youtube embeds on recent (2019?) squarespace sites fail to download

[galgeek]

Checklist

• I'm reporting a new site support request
• I've verified that I'm running youtube-dl version 2019.07.16
• I've checked that all provided URLs are alive and playable in a browser
• I've checked that none of provided URLs violate any copyrights
• I've searched the bugtracker for similar site support requests including closed ones

Description

youtube-dl fails on youtube embeds on some recent squarespace sites, including
http://www.ootboxford.com/
https://www.harvardballetcompany.org/past-productions
http://www.immediategratification.org/

youtube-dl --write-pages http://www.ootboxford.com
produces a dump showing the site's youtube embed like so:

<div class="sqs-block video-block sqs-block-video" data-block-json="&#123;&quot;layout&quot;:&quot;caption-hidden&quot;,&quot;overlay&quot;:false,&quot;description&quot;:&#123;&quot;html&quot;:&quot;&lt;p&gt;Long-standing supporters of Helen &amp;amp; Douglas House, Out of the Blue performed at the last Childish Things - here they are, talking about the show, and their involvement with the charity.&lt;/p&gt;&quot;,&quot;source&quot;:&quot;&lt;p&gt;Long-standing supporters of Helen &amp;amp; Douglas House, Out of the Blue performed at the last Childish Things - here they are, talking about the show, and their involvement with the charity.&lt;/p&gt;&quot;&#125;,&quot;hSize&quot;:null,&quot;floatDir&quot;:null,&quot;html&quot;:&quot;&lt;iframe src=\&quot;//www.youtube.com/embed/Tc7b_JGdZfw?wmode=opaque&amp;amp;enablejsapi=1\&quot; height=\&quot;480\&quot; width=\&quot;854\&quot; scrolling=\&quot;no\&quot; frameborder=\&quot;0\&quot; allowfullscreen=\&quot;\&quot;&gt;\n&lt;/iframe&gt;&quot;,&quot;url&quot;:&quot;https://www.youtube.com/watch?v=Tc7b_JGdZfw&quot;,&quot;width&quot;:854,&quot;height&quot;:480,&quot;providerName&quot;:&quot;YouTube&quot;,&quot;thumbnailUrl&quot;:&quot;https://i.ytimg.com/vi/Tc7b_JGdZfw/hqdefault.jpg&quot;,&quot;resolvedBy&quot;:&quot;youtube&quot;&#125;" data-block-type="32" id="block-yui_3_17_2_22_1450272268012_7317">

youtube-dl -v http://www.ootboxford.com
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', 'http://www.ootboxford.com']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2019.07.16
[debug] Python version 3.5.6 (CPython) - Darwin-16.7.0-x86_64-i386-64bit
[debug] exe versions: none
[debug] Proxy map: {}
[generic] www.ootboxford: Requesting header
WARNING: Falling back on generic information extractor.
[generic] www.ootboxford: Downloading webpage
[generic] www.ootboxford: Extracting information
ERROR: Unsupported URL: http://www.ootboxford.com
Traceback (most recent call last):
  File "/1/broz-venv/lib/python3.5/site-packages/youtube_dl/YoutubeDL.py", line 796, in extract_info
    ie_result = ie.extract(url)
  File "/1/broz-venv/lib/python3.5/site-packages/youtube_dl/extractor/common.py", line 530, in extract
    ie_result = self._real_extract(url)
  File "/1/broz-venv/lib/python3.5/site-packages/youtube_dl/extractor/generic.py", line 3333, in _real_extract
    raise UnsupportedError(url)
youtube_dl.utils.UnsupportedError: Unsupported URL: http://www.ootboxford.com

[galgeek]

This issue is still present in youtube-dl version 2019.08.13.

$ youtube-dl -v http://ootboxford.com
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['-v', 'http://ootboxford.com']
[debug] Encodings: locale UTF-8, fs utf-8, out UTF-8, pref UTF-8
[debug] youtube-dl version 2019.08.13
[debug] Python version 3.5.6 (CPython) - Darwin-16.7.0-x86_64-i386-64bit
[debug] exe versions: none
[debug] Proxy map: {}
[generic] ootboxford: Requesting header
[redirect] Following redirect to http://www.ootboxford.com/
[generic] www.ootboxford: Requesting header
WARNING: Falling back on generic information extractor.
[generic] www.ootboxford: Downloading webpage
[generic] www.ootboxford: Extracting information
ERROR: Unsupported URL: http://www.ootboxford.com/
Traceback (most recent call last):
  File "/1/broz-venv/lib/python3.5/site-packages/youtube_dl/YoutubeDL.py", line 796, in extract_info
    ie_result = ie.extract(url)
  File "/1/broz-venv/lib/python3.5/site-packages/youtube_dl/extractor/common.py", line 530, in extract
    ie_result = self._real_extract(url)
  File "/1/broz-venv/lib/python3.5/site-packages/youtube_dl/extractor/generic.py", line 3333, in _real_extract
    raise UnsupportedError(url)
youtube_dl.utils.UnsupportedError: Unsupported URL: http://www.ootboxford.com/