T176353: Parsoid closes unclosed <del> tag while Tidy doesn't highlighted one more source of differing behavior between a HTML4 and HTML5 parser.
But, this is a more general problem that is not limited to the <del> tag. Based on my understanding of the HTML5 spec, the following set of tags are going to be affected similarl: HTML5 tags - HTML4 block tags - HTML5 formatting tags - HTML5 void tags - those that won't be sanitized away - a few others.
Some of the tags in this category are: abbr, cite, del, sub, sup, span.
So, the linting rule is: If we have a tag in the above set, and it is auto-closed, then, page rendering is changed when Tidy is replaced.
Right now, this behavior is already tracked by Linter, but it will likely show up in the misnested-tag or in missing-end-tag categories. This ticket is a proposal to surface these in a category of its own since this will actually affect rendering.