diff --git a/LayoutTests/imported/w3c/ChangeLog b/LayoutTests/imported/w3c/ChangeLog
index 787554e66abef..4afd8d49d9c9b 100644
--- a/LayoutTests/imported/w3c/ChangeLog
+++ b/LayoutTests/imported/w3c/ChangeLog
@@ -1,3 +1,15 @@
+2020-10-02  Alex Christensen  <achristensen@webkit.org>
+
+        Copy host from base file URL
+        https://bugs.webkit.org/show_bug.cgi?id=217170
+
+        Reviewed by Brady Eidson.
+
+        * web-platform-tests/url/a-element-expected.txt:
+        * web-platform-tests/url/a-element-xhtml-expected.txt:
+        * web-platform-tests/url/resources/urltestdata.json:
+        * web-platform-tests/url/url-constructor-expected.txt:
+
 2020-10-02  Sam Weinig  <weinig@apple.com>
 
         Re-align HTMLElement with the HTML spec
diff --git a/LayoutTests/imported/w3c/web-platform-tests/url/a-element-expected.txt b/LayoutTests/imported/w3c/web-platform-tests/url/a-element-expected.txt
index 38411ef39d1d3..feb1889745862 100644
--- a/LayoutTests/imported/w3c/web-platform-tests/url/a-element-expected.txt
+++ b/LayoutTests/imported/w3c/web-platform-tests/url/a-element-expected.txt
@@ -417,6 +417,8 @@ PASS Parsing: <pix/submit.gif> against <file:///C:/Users/Domenic/Dropbox/GitHub/
 PASS Parsing: <..> against <file:///C:/>
 PASS Parsing: <..> against <file:///>
 PASS Parsing: </> against <file:///C:/a/b>
+PASS Parsing: </> against <file://h/C:/a/b>
+PASS Parsing: </> against <file://h/a/b>
 PASS Parsing: <//d:> against <file:///C:/a/b>
 PASS Parsing: <//d:/..> against <file:///C:/a/b>
 PASS Parsing: <..> against <file:///ab:/>
@@ -427,24 +429,24 @@ PASS Parsing: <?x> against <file:///test?test#test>
 PASS Parsing: <file:?x> against <file:///test?test#test>
 PASS Parsing: <#x> against <file:///test?test#test>
 PASS Parsing: <file:#x> against <file:///test?test#test>
-FAIL Parsing: <file:\\//> against <about:blank> assert_equals: href expected "file:///" but got "file:////"
-FAIL Parsing: <file:\\\\> against <about:blank> assert_equals: href expected "file:///" but got "file:////"
-FAIL Parsing: <file:\\\\?fox> against <about:blank> assert_equals: href expected "file:///?fox" but got "file:////?fox"
-FAIL Parsing: <file:\\\\#guppy> against <about:blank> assert_equals: href expected "file:///#guppy" but got "file:////#guppy"
-FAIL Parsing: <file://spider///> against <about:blank> assert_equals: href expected "file://spider/" but got "file://spider///"
-FAIL Parsing: <file:\\localhost//> against <about:blank> assert_equals: href expected "file:///" but got "file:////"
+PASS Parsing: <file:\\//> against <about:blank>
+PASS Parsing: <file:\\\\> against <about:blank>
+PASS Parsing: <file:\\\\?fox> against <about:blank>
+PASS Parsing: <file:\\\\#guppy> against <about:blank>
+PASS Parsing: <file://spider///> against <about:blank>
+PASS Parsing: <file:\\localhost//> against <about:blank>
 PASS Parsing: <file:///localhost//cat> against <about:blank>
-FAIL Parsing: <file://\/localhost//cat> against <about:blank> assert_equals: href expected "file:///localhost//cat" but got "file:////localhost//cat"
-FAIL Parsing: <file://localhost//a//../..//> against <about:blank> assert_equals: href expected "file:///" but got "file://///"
-FAIL Parsing: </////mouse> against <file:///elephant> assert_equals: href expected "file:///mouse" but got "file://///mouse"
+PASS Parsing: <file://\/localhost//cat> against <about:blank>
+PASS Parsing: <file://localhost//a//../..//> against <about:blank>
+PASS Parsing: </////mouse> against <file:///elephant>
 PASS Parsing: <\//pig> against <file://lion/>
-FAIL Parsing: <\/localhost//pig> against <file://lion/> assert_equals: href expected "file:///pig" but got "file:////pig"
-FAIL Parsing: <//localhost//pig> against <file://lion/> assert_equals: href expected "file:///pig" but got "file:////pig"
-FAIL Parsing: </..//localhost//pig> against <file://lion/> assert_equals: href expected "file://lion/localhost//pig" but got "file:////localhost//pig"
+PASS Parsing: <\/localhost//pig> against <file://lion/>
+PASS Parsing: <//localhost//pig> against <file://lion/>
+PASS Parsing: </..//localhost//pig> against <file://lion/>
 PASS Parsing: <file://> against <file://ape/>
-FAIL Parsing: </rooibos> against <file://tea/> assert_equals: href expected "file://tea/rooibos" but got "file:///rooibos"
-FAIL Parsing: </?chai> against <file://tea/> assert_equals: href expected "file://tea/?chai" but got "file:///?chai"
-FAIL Parsing: <C|> against <file://host/dir/file> assert_equals: href expected "file:///C:" but got "file://host/dir/C|"
+PASS Parsing: </rooibos> against <file://tea/>
+PASS Parsing: </?chai> against <file://tea/>
+FAIL Parsing: <C|> against <file://host/dir/file> assert_equals: href expected "file://host/C:" but got "file://host/dir/C|"
 PASS Parsing: <C|#> against <file://host/dir/file>
 PASS Parsing: <C|?> against <file://host/dir/file>
 PASS Parsing: <C|/> against <file://host/dir/file>
@@ -457,9 +459,9 @@ PASS Parsing: </c:/foo/bar> against <file:///c:/baz/qux>
 PASS Parsing: </c|/foo/bar> against <file:///c:/baz/qux>
 PASS Parsing: <file:\c:\foo\bar> against <file:///c:/baz/qux>
 PASS Parsing: </c:/foo/bar> against <file://host/path>
-FAIL Parsing: <file://example.net/C:/> against <about:blank> assert_equals: href expected "file:///C:/" but got "file://example.net/C:/"
-FAIL Parsing: <file://1.2.3.4/C:/> against <about:blank> assert_equals: href expected "file:///C:/" but got "file://1.2.3.4/C:/"
-FAIL Parsing: <file://[1::8]/C:/> against <about:blank> assert_equals: href expected "file:///C:/" but got "file://[1::8]/C:/"
+PASS Parsing: <file://example.net/C:/> against <about:blank>
+PASS Parsing: <file://1.2.3.4/C:/> against <about:blank>
+PASS Parsing: <file://[1::8]/C:/> against <about:blank>
 PASS Parsing: <file:/C|/> against <about:blank>
 PASS Parsing: <file://C|/> against <about:blank>
 PASS Parsing: <file:> against <about:blank>
@@ -475,6 +477,15 @@ PASS Parsing: <file:///y:/> against <about:blank>
 PASS Parsing: <file:///./y> against <about:blank>
 PASS Parsing: <file:///./y:> against <about:blank>
 PASS Parsing: <\\\.\y:> against <about:blank>
+PASS Parsing: <file://localhost//a//../..//foo> against <about:blank>
+PASS Parsing: <file://localhost////foo> against <about:blank>
+PASS Parsing: <file:////foo> against <about:blank>
+PASS Parsing: <file:///one/two> against <file:///>
+PASS Parsing: <file:////one/two> against <file:///>
+PASS Parsing: <//one/two> against <file:///>
+PASS Parsing: <///one/two> against <file:///>
+PASS Parsing: <////one/two> against <file:///>
+PASS Parsing: <file:///.//> against <file:////>
 PASS Parsing: <http://[1:0::]> against <http://example.net/>
 PASS Parsing: <http://[0:1:2:3:4:5:6:7:8]> against <http://example.net/>
 PASS Parsing: <https://[0::0::0]> against <about:blank>
diff --git a/LayoutTests/imported/w3c/web-platform-tests/url/a-element-xhtml-expected.txt b/LayoutTests/imported/w3c/web-platform-tests/url/a-element-xhtml-expected.txt
index 38411ef39d1d3..feb1889745862 100644
--- a/LayoutTests/imported/w3c/web-platform-tests/url/a-element-xhtml-expected.txt
+++ b/LayoutTests/imported/w3c/web-platform-tests/url/a-element-xhtml-expected.txt
@@ -417,6 +417,8 @@ PASS Parsing: <pix/submit.gif> against <file:///C:/Users/Domenic/Dropbox/GitHub/
 PASS Parsing: <..> against <file:///C:/>
 PASS Parsing: <..> against <file:///>
 PASS Parsing: </> against <file:///C:/a/b>
+PASS Parsing: </> against <file://h/C:/a/b>
+PASS Parsing: </> against <file://h/a/b>
 PASS Parsing: <//d:> against <file:///C:/a/b>
 PASS Parsing: <//d:/..> against <file:///C:/a/b>
 PASS Parsing: <..> against <file:///ab:/>
@@ -427,24 +429,24 @@ PASS Parsing: <?x> against <file:///test?test#test>
 PASS Parsing: <file:?x> against <file:///test?test#test>
 PASS Parsing: <#x> against <file:///test?test#test>
 PASS Parsing: <file:#x> against <file:///test?test#test>
-FAIL Parsing: <file:\\//> against <about:blank> assert_equals: href expected "file:///" but got "file:////"
-FAIL Parsing: <file:\\\\> against <about:blank> assert_equals: href expected "file:///" but got "file:////"
-FAIL Parsing: <file:\\\\?fox> against <about:blank> assert_equals: href expected "file:///?fox" but got "file:////?fox"
-FAIL Parsing: <file:\\\\#guppy> against <about:blank> assert_equals: href expected "file:///#guppy" but got "file:////#guppy"
-FAIL Parsing: <file://spider///> against <about:blank> assert_equals: href expected "file://spider/" but got "file://spider///"
-FAIL Parsing: <file:\\localhost//> against <about:blank> assert_equals: href expected "file:///" but got "file:////"
+PASS Parsing: <file:\\//> against <about:blank>
+PASS Parsing: <file:\\\\> against <about:blank>
+PASS Parsing: <file:\\\\?fox> against <about:blank>
+PASS Parsing: <file:\\\\#guppy> against <about:blank>
+PASS Parsing: <file://spider///> against <about:blank>
+PASS Parsing: <file:\\localhost//> against <about:blank>
 PASS Parsing: <file:///localhost//cat> against <about:blank>
-FAIL Parsing: <file://\/localhost//cat> against <about:blank> assert_equals: href expected "file:///localhost//cat" but got "file:////localhost//cat"
-FAIL Parsing: <file://localhost//a//../..//> against <about:blank> assert_equals: href expected "file:///" but got "file://///"
-FAIL Parsing: </////mouse> against <file:///elephant> assert_equals: href expected "file:///mouse" but got "file://///mouse"
+PASS Parsing: <file://\/localhost//cat> against <about:blank>
+PASS Parsing: <file://localhost//a//../..//> against <about:blank>
+PASS Parsing: </////mouse> against <file:///elephant>
 PASS Parsing: <\//pig> against <file://lion/>
-FAIL Parsing: <\/localhost//pig> against <file://lion/> assert_equals: href expected "file:///pig" but got "file:////pig"
-FAIL Parsing: <//localhost//pig> against <file://lion/> assert_equals: href expected "file:///pig" but got "file:////pig"
-FAIL Parsing: </..//localhost//pig> against <file://lion/> assert_equals: href expected "file://lion/localhost//pig" but got "file:////localhost//pig"
+PASS Parsing: <\/localhost//pig> against <file://lion/>
+PASS Parsing: <//localhost//pig> against <file://lion/>
+PASS Parsing: </..//localhost//pig> against <file://lion/>
 PASS Parsing: <file://> against <file://ape/>
-FAIL Parsing: </rooibos> against <file://tea/> assert_equals: href expected "file://tea/rooibos" but got "file:///rooibos"
-FAIL Parsing: </?chai> against <file://tea/> assert_equals: href expected "file://tea/?chai" but got "file:///?chai"
-FAIL Parsing: <C|> against <file://host/dir/file> assert_equals: href expected "file:///C:" but got "file://host/dir/C|"
+PASS Parsing: </rooibos> against <file://tea/>
+PASS Parsing: </?chai> against <file://tea/>
+FAIL Parsing: <C|> against <file://host/dir/file> assert_equals: href expected "file://host/C:" but got "file://host/dir/C|"
 PASS Parsing: <C|#> against <file://host/dir/file>
 PASS Parsing: <C|?> against <file://host/dir/file>
 PASS Parsing: <C|/> against <file://host/dir/file>
@@ -457,9 +459,9 @@ PASS Parsing: </c:/foo/bar> against <file:///c:/baz/qux>
 PASS Parsing: </c|/foo/bar> against <file:///c:/baz/qux>
 PASS Parsing: <file:\c:\foo\bar> against <file:///c:/baz/qux>
 PASS Parsing: </c:/foo/bar> against <file://host/path>
-FAIL Parsing: <file://example.net/C:/> against <about:blank> assert_equals: href expected "file:///C:/" but got "file://example.net/C:/"
-FAIL Parsing: <file://1.2.3.4/C:/> against <about:blank> assert_equals: href expected "file:///C:/" but got "file://1.2.3.4/C:/"
-FAIL Parsing: <file://[1::8]/C:/> against <about:blank> assert_equals: href expected "file:///C:/" but got "file://[1::8]/C:/"
+PASS Parsing: <file://example.net/C:/> against <about:blank>
+PASS Parsing: <file://1.2.3.4/C:/> against <about:blank>
+PASS Parsing: <file://[1::8]/C:/> against <about:blank>
 PASS Parsing: <file:/C|/> against <about:blank>
 PASS Parsing: <file://C|/> against <about:blank>
 PASS Parsing: <file:> against <about:blank>
@@ -475,6 +477,15 @@ PASS Parsing: <file:///y:/> against <about:blank>
 PASS Parsing: <file:///./y> against <about:blank>
 PASS Parsing: <file:///./y:> against <about:blank>
 PASS Parsing: <\\\.\y:> against <about:blank>
+PASS Parsing: <file://localhost//a//../..//foo> against <about:blank>
+PASS Parsing: <file://localhost////foo> against <about:blank>
+PASS Parsing: <file:////foo> against <about:blank>
+PASS Parsing: <file:///one/two> against <file:///>
+PASS Parsing: <file:////one/two> against <file:///>
+PASS Parsing: <//one/two> against <file:///>
+PASS Parsing: <///one/two> against <file:///>
+PASS Parsing: <////one/two> against <file:///>
+PASS Parsing: <file:///.//> against <file:////>
 PASS Parsing: <http://[1:0::]> against <http://example.net/>
 PASS Parsing: <http://[0:1:2:3:4:5:6:7:8]> against <http://example.net/>
 PASS Parsing: <https://[0::0::0]> against <about:blank>
diff --git a/LayoutTests/imported/w3c/web-platform-tests/url/resources/urltestdata.json b/LayoutTests/imported/w3c/web-platform-tests/url/resources/urltestdata.json
index c7f7a9ed9d0b9..17ec849526e31 100644
--- a/LayoutTests/imported/w3c/web-platform-tests/url/resources/urltestdata.json
+++ b/LayoutTests/imported/w3c/web-platform-tests/url/resources/urltestdata.json
@@ -5245,6 +5245,34 @@
     "search": "",
     "hash": ""
   },
+  {
+    "input": "/",
+    "base": "file://h/C:/a/b",
+    "href": "file://h/C:/",
+    "protocol": "file:",
+    "username": "",
+    "password": "",
+    "host": "h",
+    "hostname": "h",
+    "port": "",
+    "pathname": "/C:/",
+    "search": "",
+    "hash": ""
+  },
+  {
+    "input": "/",
+    "base": "file://h/a/b",
+    "href": "file://h/",
+    "protocol": "file:",
+    "username": "",
+    "password": "",
+    "host": "h",
+    "hostname": "h",
+    "port": "",
+    "pathname": "/",
+    "search": "",
+    "hash": ""
+  },
   {
     "input": "//d:",
     "base": "file:///C:/a/b",
@@ -5389,84 +5417,84 @@
   {
     "input": "file:\\\\//",
     "base": "about:blank",
-    "href": "file:///",
+    "href": "file:////",
     "protocol": "file:",
     "username": "",
     "password": "",
     "host": "",
     "hostname": "",
     "port": "",
-    "pathname": "/",
+    "pathname": "//",
     "search": "",
     "hash": ""
   },
   {
     "input": "file:\\\\\\\\",
     "base": "about:blank",
-    "href": "file:///",
+    "href": "file:////",
     "protocol": "file:",
     "username": "",
     "password": "",
     "host": "",
     "hostname": "",
     "port": "",
-    "pathname": "/",
+    "pathname": "//",
     "search": "",
     "hash": ""
   },
   {
     "input": "file:\\\\\\\\?fox",
     "base": "about:blank",
-    "href": "file:///?fox",
+    "href": "file:////?fox",
     "protocol": "file:",
     "username": "",
     "password": "",
     "host": "",
     "hostname": "",
     "port": "",
-    "pathname": "/",
+    "pathname": "//",
     "search": "?fox",
     "hash": ""
   },
   {
     "input": "file:\\\\\\\\#guppy",
     "base": "about:blank",
-    "href": "file:///#guppy",
+    "href": "file:////#guppy",
     "protocol": "file:",
     "username": "",
     "password": "",
     "host": "",
     "hostname": "",
     "port": "",
-    "pathname": "/",
+    "pathname": "//",
     "search": "",
     "hash": "#guppy"
   },
   {
     "input": "file://spider///",
     "base": "about:blank",
-    "href": "file://spider/",
+    "href": "file://spider///",
     "protocol": "file:",
     "username": "",
     "password": "",
     "host": "spider",
     "hostname": "spider",
     "port": "",
-    "pathname": "/",
+    "pathname": "///",
     "search": "",
     "hash": ""
   },
   {
     "input": "file:\\\\localhost//",
     "base": "about:blank",
-    "href": "file:///",
+    "href": "file:////",
     "protocol": "file:",
     "username": "",
     "password": "",
     "host": "",
     "hostname": "",
     "port": "",
-    "pathname": "/",
+    "pathname": "//",
     "search": "",
     "hash": ""
   },
@@ -5487,42 +5515,42 @@
   {
     "input": "file://\\/localhost//cat",
     "base": "about:blank",
-    "href": "file:///localhost//cat",
+    "href": "file:////localhost//cat",
     "protocol": "file:",
     "username": "",
     "password": "",
     "host": "",
     "hostname": "",
     "port": "",
-    "pathname": "/localhost//cat",
+    "pathname": "//localhost//cat",
     "search": "",
     "hash": ""
   },
   {
     "input": "file://localhost//a//../..//",
     "base": "about:blank",
-    "href": "file:///",
+    "href": "file://///",
     "protocol": "file:",
     "username": "",
     "password": "",
     "host": "",
     "hostname": "",
     "port": "",
-    "pathname": "/",
+    "pathname": "///",
     "search": "",
     "hash": ""
   },
   {
     "input": "/////mouse",
     "base": "file:///elephant",
-    "href": "file:///mouse",
+    "href": "file://///mouse",
     "protocol": "file:",
     "username": "",
     "password": "",
     "host": "",
     "hostname": "",
     "port": "",
-    "pathname": "/mouse",
+    "pathname": "///mouse",
     "search": "",
     "hash": ""
   },
@@ -5543,42 +5571,42 @@
   {
     "input": "\\/localhost//pig",
     "base": "file://lion/",
-    "href": "file:///pig",
+    "href": "file:////pig",
     "protocol": "file:",
     "username": "",
     "password": "",
     "host": "",
     "hostname": "",
     "port": "",
-    "pathname": "/pig",
+    "pathname": "//pig",
     "search": "",
     "hash": ""
   },
   {
     "input": "//localhost//pig",
     "base": "file://lion/",
-    "href": "file:///pig",
+    "href": "file:////pig",
     "protocol": "file:",
     "username": "",
     "password": "",
     "host": "",
     "hostname": "",
     "port": "",
-    "pathname": "/pig",
+    "pathname": "//pig",
     "search": "",
     "hash": ""
   },
   {
     "input": "/..//localhost//pig",
     "base": "file://lion/",
-    "href": "file://lion/localhost//pig",
+    "href": "file://lion//localhost//pig",
     "protocol": "file:",
     "username": "",
     "password": "",
     "host": "lion",
     "hostname": "lion",
     "port": "",
-    "pathname": "/localhost//pig",
+    "pathname": "//localhost//pig",
     "search": "",
     "hash": ""
   },
@@ -5629,12 +5657,12 @@
   {
     "input": "C|",
     "base": "file://host/dir/file",
-    "href": "file:///C:",
+    "href": "file://host/C:",
     "protocol": "file:",
     "username": "",
     "password": "",
-    "host": "",
-    "hostname": "",
+    "host": "host",
+    "hostname": "host",
     "port": "",
     "pathname": "/C:",
     "search": "",
@@ -5643,12 +5671,12 @@
   {
     "input": "C|#",
     "base": "file://host/dir/file",
-    "href": "file:///C:#",
+    "href": "file://host/C:#",
     "protocol": "file:",
     "username": "",
     "password": "",
-    "host": "",
-    "hostname": "",
+    "host": "host",
+    "hostname": "host",
     "port": "",
     "pathname": "/C:",
     "search": "",
@@ -5657,12 +5685,12 @@
   {
     "input": "C|?",
     "base": "file://host/dir/file",
-    "href": "file:///C:?",
+    "href": "file://host/C:?",
     "protocol": "file:",
     "username": "",
     "password": "",
-    "host": "",
-    "hostname": "",
+    "host": "host",
+    "hostname": "host",
     "port": "",
     "pathname": "/C:",
     "search": "",
@@ -5671,12 +5699,12 @@
   {
     "input": "C|/",
     "base": "file://host/dir/file",
-    "href": "file:///C:/",
+    "href": "file://host/C:/",
     "protocol": "file:",
     "username": "",
     "password": "",
-    "host": "",
-    "hostname": "",
+    "host": "host",
+    "hostname": "host",
     "port": "",
     "pathname": "/C:/",
     "search": "",
@@ -5685,12 +5713,12 @@
   {
     "input": "C|\n/",
     "base": "file://host/dir/file",
-    "href": "file:///C:/",
+    "href": "file://host/C:/",
     "protocol": "file:",
     "username": "",
     "password": "",
-    "host": "",
-    "hostname": "",
+    "host": "host",
+    "hostname": "host",
     "port": "",
     "pathname": "/C:/",
     "search": "",
@@ -5699,12 +5727,12 @@
   {
     "input": "C|\\",
     "base": "file://host/dir/file",
-    "href": "file:///C:/",
+    "href": "file://host/C:/",
     "protocol": "file:",
     "username": "",
     "password": "",
-    "host": "",
-    "hostname": "",
+    "host": "host",
+    "hostname": "host",
     "port": "",
     "pathname": "/C:/",
     "search": "",
@@ -5784,27 +5812,27 @@
   {
     "input": "/c:/foo/bar",
     "base": "file://host/path",
-    "href": "file:///c:/foo/bar",
+    "href": "file://host/c:/foo/bar",
     "protocol": "file:",
     "username": "",
     "password": "",
-    "host": "",
-    "hostname": "",
+    "host": "host",
+    "hostname": "host",
     "port": "",
     "pathname": "/c:/foo/bar",
     "search": "",
     "hash": ""
   },
-  "# Windows drive letter quirk with not empty host",
+  "# Do not drop the host in the presence of a drive letter",
   {
     "input": "file://example.net/C:/",
     "base": "about:blank",
-    "href": "file:///C:/",
+    "href": "file://example.net/C:/",
     "protocol": "file:",
     "username": "",
     "password": "",
-    "host": "",
-    "hostname": "",
+    "host": "example.net",
+    "hostname": "example.net",
     "port": "",
     "pathname": "/C:/",
     "search": "",
@@ -5813,12 +5841,12 @@
   {
     "input": "file://1.2.3.4/C:/",
     "base": "about:blank",
-    "href": "file:///C:/",
+    "href": "file://1.2.3.4/C:/",
     "protocol": "file:",
     "username": "",
     "password": "",
-    "host": "",
-    "hostname": "",
+    "host": "1.2.3.4",
+    "hostname": "1.2.3.4",
     "port": "",
     "pathname": "/C:/",
     "search": "",
@@ -5827,12 +5855,12 @@
   {
     "input": "file://[1::8]/C:/",
     "base": "about:blank",
-    "href": "file:///C:/",
+    "href": "file://[1::8]/C:/",
     "protocol": "file:",
     "username": "",
     "password": "",
-    "host": "",
-    "hostname": "",
+    "host": "[1::8]",
+    "hostname": "[1::8]",
     "port": "",
     "pathname": "/C:/",
     "search": "",
@@ -6034,6 +6062,133 @@
     "base": "about:blank",
     "failure": true
   },
+  "# Additional file URL tetsts for (https://github.com/whatwg/url/issues/405)",
+  {
+    "input": "file://localhost//a//../..//foo",
+    "base": "about:blank",
+    "href": "file://///foo",
+    "protocol": "file:",
+    "username": "",
+    "password": "",
+    "host": "",
+    "hostname": "",
+    "port": "",
+    "pathname": "///foo",
+    "search": "",
+    "hash": ""
+  },
+  {
+    "input": "file://localhost////foo",
+    "base": "about:blank",
+    "href": "file://////foo",
+    "protocol": "file:",
+    "username": "",
+    "password": "",
+    "host": "",
+    "hostname": "",
+    "port": "",
+    "pathname": "////foo",
+    "search": "",
+    "hash": ""
+  },
+  {
+    "input": "file:////foo",
+    "base": "about:blank",
+    "href": "file:////foo",
+    "protocol": "file:",
+    "username": "",
+    "password": "",
+    "host": "",
+    "hostname": "",
+    "port": "",
+    "pathname": "//foo",
+    "search": "",
+    "hash": ""
+  },
+  {
+    "input": "file:///one/two",
+    "base": "file:///",
+    "href": "file:///one/two",
+    "protocol": "file:",
+    "username": "",
+    "password": "",
+    "host": "",
+    "hostname": "",
+    "port": "",
+    "pathname": "/one/two",
+    "search": "",
+    "hash": ""
+  },
+  {
+    "input": "file:////one/two",
+    "base": "file:///",
+    "href": "file:////one/two",
+    "protocol": "file:",
+    "username": "",
+    "password": "",
+    "host": "",
+    "hostname": "",
+    "port": "",
+    "pathname": "//one/two",
+    "search": "",
+    "hash": ""
+  },
+  {
+    "input": "//one/two",
+    "base": "file:///",
+    "href": "file://one/two",
+    "protocol": "file:",
+    "username": "",
+    "password": "",
+    "host": "one",
+    "hostname": "one",
+    "port": "",
+    "pathname": "/two",
+    "search": "",
+    "hash": ""
+  },
+  {
+    "input": "///one/two",
+    "base": "file:///",
+    "href": "file:///one/two",
+    "protocol": "file:",
+    "username": "",
+    "password": "",
+    "host": "",
+    "hostname": "",
+    "port": "",
+    "pathname": "/one/two",
+    "search": "",
+    "hash": ""
+  },
+  {
+    "input": "////one/two",
+    "base": "file:///",
+    "href": "file:////one/two",
+    "protocol": "file:",
+    "username": "",
+    "password": "",
+    "host": "",
+    "hostname": "",
+    "port": "",
+    "pathname": "//one/two",
+    "search": "",
+    "hash": ""
+  },
+  {
+    "input": "file:///.//",
+    "base": "file:////",
+    "href": "file:////",
+    "protocol": "file:",
+    "username": "",
+    "password": "",
+    "host": "",
+    "hostname": "",
+    "port": "",
+    "pathname": "//",
+    "search": "",
+    "hash": ""
+  },
   "# IPv6 tests",
   {
     "input": "http://[1:0::]",
@@ -7217,4 +7372,4 @@
     "search": "",
     "username": "user"
   }
-]
\ No newline at end of file
+]
diff --git a/LayoutTests/imported/w3c/web-platform-tests/url/url-constructor-expected.txt b/LayoutTests/imported/w3c/web-platform-tests/url/url-constructor-expected.txt
index 86bf240e20982..ce1051af8356c 100644
--- a/LayoutTests/imported/w3c/web-platform-tests/url/url-constructor-expected.txt
+++ b/LayoutTests/imported/w3c/web-platform-tests/url/url-constructor-expected.txt
@@ -420,6 +420,8 @@ PASS Parsing: <pix/submit.gif> against <file:///C:/Users/Domenic/Dropbox/GitHub/
 PASS Parsing: <..> against <file:///C:/>
 PASS Parsing: <..> against <file:///>
 PASS Parsing: </> against <file:///C:/a/b>
+PASS Parsing: </> against <file://h/C:/a/b>
+PASS Parsing: </> against <file://h/a/b>
 PASS Parsing: <//d:> against <file:///C:/a/b>
 PASS Parsing: <//d:/..> against <file:///C:/a/b>
 PASS Parsing: <..> against <file:///ab:/>
@@ -430,24 +432,24 @@ PASS Parsing: <?x> against <file:///test?test#test>
 PASS Parsing: <file:?x> against <file:///test?test#test>
 PASS Parsing: <#x> against <file:///test?test#test>
 PASS Parsing: <file:#x> against <file:///test?test#test>
-FAIL Parsing: <file:\\//> against <about:blank> assert_equals: href expected "file:///" but got "file:////"
-FAIL Parsing: <file:\\\\> against <about:blank> assert_equals: href expected "file:///" but got "file:////"
-FAIL Parsing: <file:\\\\?fox> against <about:blank> assert_equals: href expected "file:///?fox" but got "file:////?fox"
-FAIL Parsing: <file:\\\\#guppy> against <about:blank> assert_equals: href expected "file:///#guppy" but got "file:////#guppy"
-FAIL Parsing: <file://spider///> against <about:blank> assert_equals: href expected "file://spider/" but got "file://spider///"
-FAIL Parsing: <file:\\localhost//> against <about:blank> assert_equals: href expected "file:///" but got "file:////"
+PASS Parsing: <file:\\//> against <about:blank>
+PASS Parsing: <file:\\\\> against <about:blank>
+PASS Parsing: <file:\\\\?fox> against <about:blank>
+PASS Parsing: <file:\\\\#guppy> against <about:blank>
+PASS Parsing: <file://spider///> against <about:blank>
+PASS Parsing: <file:\\localhost//> against <about:blank>
 PASS Parsing: <file:///localhost//cat> against <about:blank>
-FAIL Parsing: <file://\/localhost//cat> against <about:blank> assert_equals: href expected "file:///localhost//cat" but got "file:////localhost//cat"
-FAIL Parsing: <file://localhost//a//../..//> against <about:blank> assert_equals: href expected "file:///" but got "file://///"
-FAIL Parsing: </////mouse> against <file:///elephant> assert_equals: href expected "file:///mouse" but got "file://///mouse"
+PASS Parsing: <file://\/localhost//cat> against <about:blank>
+PASS Parsing: <file://localhost//a//../..//> against <about:blank>
+PASS Parsing: </////mouse> against <file:///elephant>
 PASS Parsing: <\//pig> against <file://lion/>
-FAIL Parsing: <\/localhost//pig> against <file://lion/> assert_equals: href expected "file:///pig" but got "file:////pig"
-FAIL Parsing: <//localhost//pig> against <file://lion/> assert_equals: href expected "file:///pig" but got "file:////pig"
-FAIL Parsing: </..//localhost//pig> against <file://lion/> assert_equals: href expected "file://lion/localhost//pig" but got "file:////localhost//pig"
+PASS Parsing: <\/localhost//pig> against <file://lion/>
+PASS Parsing: <//localhost//pig> against <file://lion/>
+PASS Parsing: </..//localhost//pig> against <file://lion/>
 PASS Parsing: <file://> against <file://ape/>
-FAIL Parsing: </rooibos> against <file://tea/> assert_equals: href expected "file://tea/rooibos" but got "file:///rooibos"
-FAIL Parsing: </?chai> against <file://tea/> assert_equals: href expected "file://tea/?chai" but got "file:///?chai"
-FAIL Parsing: <C|> against <file://host/dir/file> assert_equals: href expected "file:///C:" but got "file://host/dir/C|"
+PASS Parsing: </rooibos> against <file://tea/>
+PASS Parsing: </?chai> against <file://tea/>
+FAIL Parsing: <C|> against <file://host/dir/file> assert_equals: href expected "file://host/C:" but got "file://host/dir/C|"
 PASS Parsing: <C|#> against <file://host/dir/file>
 PASS Parsing: <C|?> against <file://host/dir/file>
 PASS Parsing: <C|/> against <file://host/dir/file>
@@ -460,9 +462,9 @@ PASS Parsing: </c:/foo/bar> against <file:///c:/baz/qux>
 PASS Parsing: </c|/foo/bar> against <file:///c:/baz/qux>
 PASS Parsing: <file:\c:\foo\bar> against <file:///c:/baz/qux>
 PASS Parsing: </c:/foo/bar> against <file://host/path>
-FAIL Parsing: <file://example.net/C:/> against <about:blank> assert_equals: href expected "file:///C:/" but got "file://example.net/C:/"
-FAIL Parsing: <file://1.2.3.4/C:/> against <about:blank> assert_equals: href expected "file:///C:/" but got "file://1.2.3.4/C:/"
-FAIL Parsing: <file://[1::8]/C:/> against <about:blank> assert_equals: href expected "file:///C:/" but got "file://[1::8]/C:/"
+PASS Parsing: <file://example.net/C:/> against <about:blank>
+PASS Parsing: <file://1.2.3.4/C:/> against <about:blank>
+PASS Parsing: <file://[1::8]/C:/> against <about:blank>
 PASS Parsing: <file:/C|/> against <about:blank>
 PASS Parsing: <file://C|/> against <about:blank>
 PASS Parsing: <file:> against <about:blank>
@@ -478,6 +480,15 @@ PASS Parsing: <file:///y:/> against <about:blank>
 PASS Parsing: <file:///./y> against <about:blank>
 PASS Parsing: <file:///./y:> against <about:blank>
 PASS Parsing: <\\\.\y:> against <about:blank>
+PASS Parsing: <file://localhost//a//../..//foo> against <about:blank>
+PASS Parsing: <file://localhost////foo> against <about:blank>
+PASS Parsing: <file:////foo> against <about:blank>
+PASS Parsing: <file:///one/two> against <file:///>
+PASS Parsing: <file:////one/two> against <file:///>
+PASS Parsing: <//one/two> against <file:///>
+PASS Parsing: <///one/two> against <file:///>
+PASS Parsing: <////one/two> against <file:///>
+PASS Parsing: <file:///.//> against <file:////>
 PASS Parsing: <http://[1:0::]> against <http://example.net/>
 PASS Parsing: <http://[0:1:2:3:4:5:6:7:8]> against <http://example.net/>
 PASS Parsing: <https://[0::0::0]> against <about:blank>
diff --git a/Source/WTF/ChangeLog b/Source/WTF/ChangeLog
index 60140330d5496..b7706f3dbb942 100644
--- a/Source/WTF/ChangeLog
+++ b/Source/WTF/ChangeLog
@@ -1,3 +1,27 @@
+2020-10-02  Alex Christensen  <achristensen@webkit.org>
+
+        Copy host from base file URL
+        https://bugs.webkit.org/show_bug.cgi?id=217170
+
+        Reviewed by Brady Eidson.
+
+        This matches Chrome and the URL specification.
+        Covered by newly passing web platform tests.
+
+        I also updated the web platform tests from https://github.com/web-platform-tests/wpt/pull/25716
+        which aligns with Safari in cases except copying of the host from base file URLs.
+
+        The implementation pushes copying from the base URL downstream in the parsing process to where it is in the URL specification
+        so that we can properly decide how much of the base URL to copy and so we can copy it into the right place in the result URL.
+
+        I also updated an assertion that makes sure that we re-use the input String if possible because there are cases where we copy
+        part of the parent URL, which is a "syntax violation" (meaning we copy the string parts and assemble a new one), then re-assemble
+        a new String that is equal to the input string.  This is not a problem, it just needed to be reflected in the assertion.
+
+        * wtf/URLParser.cpp:
+        (WTF::URLParser::URLParser):
+        (WTF::URLParser::parse):
+
 2020-10-02  Youenn Fablet  <youenn@apple.com>
 
         Add AVAssetWriter SPI header
diff --git a/Source/WTF/wtf/URLParser.cpp b/Source/WTF/wtf/URLParser.cpp
index 6003fe2c00438..52ee6e55a637d 100644
--- a/Source/WTF/wtf/URLParser.cpp
+++ b/Source/WTF/wtf/URLParser.cpp
@@ -1074,8 +1074,8 @@ URLParser::URLParser(const String& input, const URL& base, const URLTextEncoding
 
     ASSERT(!m_url.m_isValid
         || m_didSeeSyntaxViolation == (m_url.string() != input)
-        || (input.isAllSpecialCharacters<isC0ControlOrSpace>()
-            && m_url.m_string == base.m_string.left(base.m_queryEnd)));
+        || (input.isAllSpecialCharacters<isC0ControlOrSpace>() && m_url.m_string == base.m_string.left(base.m_queryEnd))
+        || (base.isValid() && base.protocolIs("file")));
     ASSERT(internalValuesConsistent(m_url));
 #if ASSERT_ENABLED
     if (!m_didSeeSyntaxViolation) {
@@ -1265,8 +1265,6 @@ void URLParser::parse(const CharacterType* input, const unsigned length, const U
                 state = State::Relative;
                 break;
             }
-            copyURLPartsUntil(base, URLPart::SchemeEnd, c, nonUTF8QueryEncoding);
-            appendToASCIIBuffer(':');
             state = State::File;
             break;
         case State::SpecialRelativeOrAuthority:
@@ -1529,13 +1527,26 @@ void URLParser::parse(const CharacterType* input, const unsigned length, const U
                 if (base.isValid() && base.protocolIs("file") && shouldCopyFileURL(c))
                     copyURLPartsUntil(base, URLPart::PathAfterLastSlash, c, nonUTF8QueryEncoding);
                 else {
-                    appendToASCIIBuffer("///", 3);
-                    m_url.m_userStart = currentPosition(c) - 1;
-                    m_url.m_userEnd = m_url.m_userStart;
-                    m_url.m_passwordEnd = m_url.m_userStart;
-                    m_url.m_hostEnd = m_url.m_userStart;
-                    m_url.m_portLength = 0;
-                    m_url.m_pathAfterLastSlash = m_url.m_userStart + 1;
+                    bool copiedHost = false;
+                    if (base.isValid() && base.protocolIs("file")) {
+                        if (base.host().isEmpty()) {
+                            copyURLPartsUntil(base, URLPart::SchemeEnd, c, nonUTF8QueryEncoding);
+                            appendToASCIIBuffer(":///", 4);
+                        } else {
+                            copyURLPartsUntil(base, URLPart::PortEnd, c, nonUTF8QueryEncoding);
+                            appendToASCIIBuffer('/');
+                            copiedHost = true;
+                        }
+                    } else
+                        appendToASCIIBuffer("///", 3);
+                    if (!copiedHost) {
+                        m_url.m_userStart = currentPosition(c) - 1;
+                        m_url.m_userEnd = m_url.m_userStart;
+                        m_url.m_passwordEnd = m_url.m_userStart;
+                        m_url.m_hostEnd = m_url.m_userStart;
+                        m_url.m_portLength = 0;
+                    }
+                    m_url.m_pathAfterLastSlash = m_url.m_hostEnd + 1;
                     if (isWindowsDriveLetter(c))
                         appendWindowsDriveLetter(c);
                 }
@@ -1548,6 +1559,10 @@ void URLParser::parse(const CharacterType* input, const unsigned length, const U
             if (LIKELY(*c == '/' || *c == '\\')) {
                 if (UNLIKELY(*c == '\\'))
                     syntaxViolation(c);
+                if (base.isValid() && base.protocolIs("file")) {
+                    copyURLPartsUntil(base, URLPart::SchemeEnd, c, nonUTF8QueryEncoding);
+                    appendToASCIIBuffer(":/", 2);
+                }
                 appendToASCIIBuffer('/');
                 advance(c);
                 m_url.m_userStart = currentPosition(c);
@@ -1559,21 +1574,37 @@ void URLParser::parse(const CharacterType* input, const unsigned length, const U
                 state = State::FileHost;
                 break;
             }
-            syntaxViolation(c);
-            appendToASCIIBuffer("//", 2);
-            m_url.m_userStart = currentPosition(c) - 1;
-            m_url.m_userEnd = m_url.m_userStart;
-            m_url.m_passwordEnd = m_url.m_userStart;
-            m_url.m_hostEnd = m_url.m_userStart;
-            m_url.m_portLength = 0;
+            {
+                bool copiedHost = false;
+                if (base.isValid() && base.protocolIs("file")) {
+                    if (base.host().isEmpty()) {
+                        copyURLPartsUntil(base, URLPart::SchemeEnd, c, nonUTF8QueryEncoding);
+                        appendToASCIIBuffer(":///", 4);
+                    } else {
+                        copyURLPartsUntil(base, URLPart::PortEnd, c, nonUTF8QueryEncoding);
+                        appendToASCIIBuffer('/');
+                        copiedHost = true;
+                    }
+                } else {
+                    syntaxViolation(c);
+                    appendToASCIIBuffer("//", 2);
+                }
+                if (!copiedHost) {
+                    m_url.m_userStart = currentPosition(c) - 1;
+                    m_url.m_userEnd = m_url.m_userStart;
+                    m_url.m_passwordEnd = m_url.m_userStart;
+                    m_url.m_hostEnd = m_url.m_userStart;
+                    m_url.m_portLength = 0;
+                }
+            }
             if (isWindowsDriveLetter(c)) {
                 appendWindowsDriveLetter(c);
-                m_url.m_pathAfterLastSlash = m_url.m_userStart + 1;
+                m_url.m_pathAfterLastSlash = m_url.m_hostEnd + 1;
             } else if (copyBaseWindowsDriveLetter(base)) {
                 appendToASCIIBuffer('/');
-                m_url.m_pathAfterLastSlash = m_url.m_userStart + 4;
+                m_url.m_pathAfterLastSlash = m_url.m_hostEnd + 4;
             } else
-                m_url.m_pathAfterLastSlash = m_url.m_userStart + 1;
+                m_url.m_pathAfterLastSlash = m_url.m_hostEnd + 1;
             state = State::Path;
             break;
         case State::FileHost:
@@ -1864,17 +1895,32 @@ void URLParser::parse(const CharacterType* input, const unsigned length, const U
     case State::FileSlash:
         LOG_FINAL_STATE("FileSlash");
         syntaxViolation(c);
-        m_url.m_userStart = currentPosition(c) + 1;
-        appendToASCIIBuffer("//", 2);
-        m_url.m_userEnd = m_url.m_userStart;
-        m_url.m_passwordEnd = m_url.m_userStart;
-        m_url.m_hostEnd = m_url.m_userStart;
-        m_url.m_portLength = 0;
+        {
+            bool copiedHost = false;
+            if (base.isValid() && base.protocolIs("file")) {
+                if (base.host().isEmpty()) {
+                    copyURLPartsUntil(base, URLPart::SchemeEnd, c, nonUTF8QueryEncoding);
+                    appendToASCIIBuffer(":/", 2);
+                } else {
+                    copyURLPartsUntil(base, URLPart::PortEnd, c, nonUTF8QueryEncoding);
+                    appendToASCIIBuffer('/');
+                    copiedHost = true;
+                }
+            }
+            if (!copiedHost) {
+                m_url.m_userStart = currentPosition(c) + 1;
+                appendToASCIIBuffer("//", 2);
+                m_url.m_userEnd = m_url.m_userStart;
+                m_url.m_passwordEnd = m_url.m_userStart;
+                m_url.m_hostEnd = m_url.m_userStart;
+                m_url.m_portLength = 0;
+            }
+        }
         if (copyBaseWindowsDriveLetter(base)) {
             appendToASCIIBuffer('/');
-            m_url.m_pathAfterLastSlash = m_url.m_userStart + 4;
+            m_url.m_pathAfterLastSlash = m_url.m_hostEnd + 4;
         } else
-            m_url.m_pathAfterLastSlash = m_url.m_userStart + 1;
+            m_url.m_pathAfterLastSlash = m_url.m_hostEnd + 1;
         m_url.m_pathEnd = m_url.m_pathAfterLastSlash;
         m_url.m_queryEnd = m_url.m_pathAfterLastSlash;
         break;
@@ -1956,7 +2002,7 @@ void URLParser::parse(const CharacterType* input, const unsigned length, const U
     } else
         m_url.m_string = String::adopt(WTFMove(m_asciiBuffer));
     m_url.m_isValid = true;
-    URL_PARSER_LOG("Parsed URL <%s>", m_url.m_string.utf8().data());
+    URL_PARSER_LOG("Parsed URL <%s>\n\n", m_url.m_string.utf8().data());
 }
 
 template<typename CharacterType>