All documentation pages failed HTML validation due to a duplicate
id="TableOfContents" (side + mobile TOC) and an invalid heading level
(h1 → h3 skip in both TOC labels and in the Resources block).

Fix:

• Replaced the <h3> labels with <p class="toc-title">/<p class="resources-title">, as already done for the footer, to remove the heading-level skip.
• Renamed the mobile TOC id to "TableOfContentsMobile" to eliminate the duplicate id.
• Updated CSS: new .toc-title/.resources-title rules use var(--size-550), bold weight, line-height 1.3 and matching paddings, so the labels keep the exact look of the previous <h3>.

Checked locally: both TOCs render as before and the pages now pass
W3C validation in this code (still they have other validation errors).