Bug report

There are several bugs which prevent generation of correct HTML:

• Comment and "processing instruction" content should not be escaped. Similar bug was fixed for XML in ElementTree ProcessingInstruction uses character entities in content #46995.
• There are more elements besides <script> and <style> whose content should not be escaped.
• <plaintext> does not have the closing tag, and its content should not be escaped.
• Empty attributes (with value None) are not supported.

Linked PRs

• gh-149489: Fix ElementTree serialization to HTML #149490
• [3.15] gh-149489: Fix ElementTree serialization to HTML (GH-149490) #150592
• [3.14] gh-149489: Fix ElementTree serialization to HTML (GH-149490) #150593
• [3.13] gh-149489: Fix ElementTree serialization to HTML (GH-149490) #150594
• [3.15] gh-149489: Fix ElementTree serialization to HTML (GH-149490) #150595
• [3.14] gh-149489: Fix ElementTree serialization to HTML (GH-149490) #150596
• [3.13] gh-149489: Fix ElementTree serialization to HTML (GH-149490) (GH-150596) #150609