[manual-of-style] Sentence case instead of Title case

Section 5.1 Document Title says:

Capitalize title words following U.S. usage.

But as part of the W3C website redesign, Studio 24 (the agency we worked with) let us know of research findings that title case is hard to read (fwiw, I concur!) and they recommended we use sentence case for titles and headings. Which we have applied to the best of our ability on new pages we had to re-create for the CMS.

I would like to extend that rule for the rest of the site, including our Specs so would like to add it to section 11.4 Case, Combining Words, and Hyphenation in the form of a new paragraph.

We would probably make this applicable starting a certain date to all future specs, and not retro-active (because there are several thousands of files to update which has in some cases hundreds of headings).

Note: There is likely other documentation for authors and editors and other "manual of style"-like documents to adjust (or to combine in one document).