a11y: add skip to main content navigation link (WCAG 2.4.1)

[mdimamhosen]

Description

This PR implements a "Skip to main content" link to satisfy WCAG 2.4.1 (Bypass Blocks, Level A).

Currently, keyboard and assistive technology users navigating react.dev must tab through all header links, search, and sidebar navigation items on every page before reaching the main article.

Changes:

• Added an accessible skip link at the top of Page.tsx using sr-only focus:not-sr-only, styled with the site's design tokens. When focused via keyboard Tab, the skip button appears at the top-left corner.
• Added id="main-content" to the <main> landmark element in Page.tsx.

Verification

• yarn tsc passed (0 errors)
• yarn prettier:diff passed
• npx next lint passed
• yarn test:eslint-local-rules passed

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[github-actions]

Size changes

Details

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

⚠️ Global Bundle Size Increased

Page	Size (compressed)
global	115.36 KB (🟡 +214 B)

Details

The global bundle is the javascript bundle that loads alongside every page. It is in its own category because its impact is much higher - an increase to its size means that every page on your website loads slower, and a decrease means every page loads faster.

Any third party scripts you have added directly to your app using the <script> tag are not accounted for in this analysis

If you want further insight into what is behind the changes, give @next/bundle-analyzer a try!

Five Pages Changed Size

The following pages changed size from the code in this PR compared to its base branch:

Page	Size (compressed)	First Load
/404	127.67 KB (🟡 +110 B)	243.03 KB
/500	127.68 KB (🟡 +110 B)	243.04 KB
/[[...markdownPath]]	130.13 KB (🟡 +110 B)	245.49 KB
/errors	127.92 KB (🟡 +110 B)	243.28 KB
/errors/[errorCode]	127.9 KB (🟡 +110 B)	243.26 KB

Details

Only the gzipped size is provided here based on an expert tip.

First Load is the size of the global bundle plus the bundle for the individual page. If a user were to show up to your website and land on a given page, the first load size represents the amount of javascript that user would need to download. If next/link is used, subsequent page loads would only need to download that page's bundle (the number in the "Size" column), since the global bundle has already been downloaded.

Any third party scripts you have added directly to your app using the <script> tag are not accounted for in this analysis

Next to the size is how much the size has increased or decreased compared with the base branch of this PR. If this percentage has increased by 10% or more, there will be a red status indicator applied, indicating that special attention should be given to this.