[Markdown] Preparing CSS docs for Markdowning

[wbamberg]

This is a tracking bug for the work we have to do to prepare the CSS documentation for Markdown conversion.

This doesn't list everything we need to do and I'll probably add some things, but gives us a list of the main items at least.

• remove id attributes except on headings (fixed by Remove css ids, final part #7529 and others)
• deal with hidden code blocks in live samples (filed as [Markdown] [CSS] Fix usages of <div class="hidden"> containing code blocks #7256)
• remove hidden elements that don't contain code blocks in a live sample
• remove summary and seoSummary class (done in removing summary and seoSummary from CSS #5894)
• remove inline styles (done in multiple PRs)
• make note/warning consistently formatted (done in Make note syntax consistent for Markdown conversion #7170 and Make warning markup consistent in CSS docs #7236)
• fix broken dd elements (done in Fix consecutive dd elements in CSS #6352)
• deal with sup and sub (filed as [Markdown] [CSS] Handle sup and sub in the CSS docs #7267)
• remove nested partial strong in code
• deal with <div class="index"> (discussed in https://github.com/mdn/content/discussions/7263, and removed in Remove index class from CSS docs #7726)
• make converter understand <pre class="hidden"> (filed as h2m converter should handle hidden on pre blocks yari#4308)
• fix live samples that use IDs which are not simple derivations of heading text (Fix live sample IDs for CSS docs #7732)
• remove random CSS classes and unsupported elements (filed as Remove css classes #7720 )

[hamishwillee]

Re "remove id attributes except on headings (#3647)"

The interesting thing is that I believe the anchor gets auto-generated if you don't specify it, and this will happen all the time the moment that we move to markdown. So possibly we should remove all ids in heading that are "correct" for the heading, and then review all the other ones to work out why they are different and fix any potential breaks.

Further, it might be nice to be able to explicitly set an anchor for some headings to prevent them moving around if the heading text changes - or at least have some way of reminding people that changing headings might break links.

[wbamberg]

Re "remove id attributes except on headings (#3647)"

The interesting thing is that I believe the anchor gets auto-generated if you don't specify it, and this will happen all the time the moment that we move to markdown. So possibly we should remove all ids in heading that are "correct" for the heading, and then review all the other ones to work out why they are different and fix any potential breaks.

Further, it might be nice to be able to explicitly set an anchor for some headings to prevent them moving around if the heading text changes - or at least have some way of reminding people that changing headings might break links.

I've started https://github.com/mdn/content/discussions/6322 for this.

[wbamberg]

I've started https://github.com/mdn/content/discussions/7263 to decide what to do about <div class="index">.