Reduce FOUT with font preloading

[julioest]

Problem:

Fonts load late, causing a flash of unstyled text (FOUT) on initial page load.

Solution:

Add head-fonts.hbs partial with Font Loading API, preload fonts, and 3-second timeout fallback to ensure fonts load before content is visible.

Changes:

• New head-fonts.hbs partial with preload links, Font Loading API implementation, and timeout logic

Impact:

Eliminates FOUT, fonts load earlier, graceful fallback for older browsers, better perceived performance.

[cppalliance-bot]

An automated preview of the documentation is available at https://545.site-docs.prtest2.cppalliance.org/index.html

If more commits are pushed to the pull request, the docs will rebuild at the same URL.

2025-12-18 21:38:08 UTC

[sdarwin]

@julioest this PR is adding "preload" on three NotoSans fonts. In the preview https://545.site-docs.prtest2.cppalliance.org/index.html -> View Source -> the "preload" is included in the source. So the modification is there, right? Nevertheless I still get FOUT in the browser. Do you?
You're welcome to merge this. If you believe it's correct.

[sdarwin]

• notosans.woff2 continues to 404. Twice.
• site.webmanifest 404's once.
• This solution is creating a dependency on the cppalliance.org website. That's alright in the short term but longer term the doc build process, using antora, should generate standalone, self-sufficient doc builds that embed the required files, and aren't contacting cppalliance.org. If possible.

Are all these "notosans" fonts the primary main fonts? If they were 404ing before, does that mean they weren't displayed? In that case, are they needed?

[julioest]

This solution is creating a dependency on the cppalliance.org website. That's alright in the short term but longer term the doc build process, using antora, should generate standalone, self-sufficient doc builds that embed the required files, and aren't contacting cppalliance.org. If possible.

100% agreed! This PR in boostlook boostorg/boostlook#147 checks for the antora-bundled font path first.

notosans.woff2 continues to 404. Twice.

It's failing because there's no /static/font/notosans.woff2 (website-v2's font path)