Reworked the validation of attribute names to better correspond to HTML standards

src/commonMain/kotlin/stream.kt

@@ -7,7 +7,7 @@ import kotlinx.html.org.w3c.dom.events.Event
 class HTMLStreamBuilder<out O : Appendable>(
     val out: O,
     val prettyPrint: Boolean,
-    val xhtmlCompatible: Boolean
+    val xhtmlCompatible: Boolean,
 ) : TagConsumer<O> {
     private var level = 0
     private var ln = true
@@ -164,23 +164,18 @@ private val escapeMap = mapOf(
     Array(maxCode + 1) { mappings[it.toChar()] }
 }
 
-private val letterRangeLowerCase = 'a'..'z'
-private val letterRangeUpperCase = 'A'..'Z'
-private val digitRange = '0'..'9'
-
-private fun Char._isLetter() = this in letterRangeLowerCase || this in letterRangeUpperCase
-private fun Char._isDigit() = this in digitRange
-
 private fun String.isValidXmlAttributeName() =
-    !startsWithXml()
-            && this.isNotEmpty()
-            && (this[0]._isLetter() || this[0] == '_')
-            && this.all { it._isLetter() || it._isDigit() || it in "._:-" }
+    this.isNotEmpty()
+        && !startsWithXml()
+        // See https://html.spec.whatwg.org/multipage/syntax.html#attributes-2 for which characters are forbidden
+        // \u000C is the form-feed character. \f is not supported in Kotlin, so it's necessary to use the
+        // unicode literal.
+        && this.none { it in "\t\n\u000C />\"'=" }
 
 private fun String.startsWithXml() = length >= 3
-        && (this[0].let { it == 'x' || it == 'X' })
-        && (this[1].let { it == 'm' || it == 'M' })
-        && (this[2].let { it == 'l' || it == 'L' })
+    && (this[0].let { it == 'x' || it == 'X' })
+    && (this[1].let { it == 'm' || it == 'M' })
+    && (this[2].let { it == 'l' || it == 'L' })
 
 internal fun Appendable.escapeAppend(value: CharSequence) {
     var lastIndex = 0

src/commonTest/kotlin/AttributesTest.kt

@@ -2,6 +2,7 @@ import kotlinx.html.div
 import kotlinx.html.stream.appendHTML
 import kotlin.test.Test
 import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
 
 class AttributesTest {
 
@@ -20,4 +21,49 @@ class AttributesTest {
         assertEquals(message, html)
         assertEquals(dataTest, dataTestAttribute)
     }
-}
+
+    @Test
+    fun testNonLetterNames() {
+        val html = buildString {
+            appendHTML(false).div {
+                attributes["[quoted_bracket]"] = "quoted_bracket"
+                attributes["(parentheses)"] = "parentheses"
+                attributes["_underscore"] = "underscore"
+                attributes["#pound"] = "pound"
+                attributes["@alpine.attr"] = "alpineAttr"
+            }
+        }
+        assertEquals(
+            """
+                <div [quoted_bracket]="quoted_bracket" (parentheses)="parentheses" _underscore="underscore" #pound="pound" @alpine.attr="alpineAttr"></div>
+            """.trimIndent(),
+            html,
+        )
+    }
+
+    @Test
+    fun testInvalidAttributeNames() {
+        listOf(
+            "", // Must not be empty
+            "XMLAttr", // Cannot start with XML
+            "xmlAttr", // That's case-insensitive btw
+            "\"", // No double quotes
+            "'", // No single quotes, either
+            "a b", // No spaces
+            "A\n", // No newline
+            "A\t", // No tab
+            "A\u000C", // No form feed
+            "A>", // No greater-than sign
+            "A/", // No forward-slash (solidus)
+            "A=", // No equals sign
+        ).forEach { attrName ->
+            assertFailsWith<IllegalArgumentException>("Invalid attribute name '$attrName' validated!") {
+                buildString {
+                    appendHTML(false).div {
+                        attributes[attrName] = "Should Fail!"
+                    }
+                }
+            }
+        }
+    }
+}