BUG: Relative URLs broken by `<base>` tag in multi-page HTML output

[NicolasRouquette]

Summary

In multi-page HTML output, the use of the <base> tag currently shifts URL resolution in a way that breaks two distinct types of resources: user-authored relative images and system-injected KaTeX assets.

This issue aims to clarify the expected behavior for these links and document where the current multi-page output diverges from that expectation.

Expected Behavior (Proposed Link Resolution Rules)

To ensure a predictable experience for Verso authors, links and assets should conceptually resolve as follows:

1. User-authored relative paths: If an author includes a relative link or image in a Markdown file (e.g., ![diagram](graphs/foo.svg)), that path should resolve relative to the location of that specific page within the project structure.
2. System-injected internal assets: Internal Verso assets (like KaTeX scripts mapped to /-verso-data/katex/...) should reliably resolve to the generated site-root directory, regardless of how deeply nested the current page is.

Reproduction

Setup:

• Verso v4.29.0, multi-page HTML output enabled.
• KaTeX feature enabled.
• A nested page created at the path: Verification/Key-Theorems/
• The Markdown content for that page includes a local image reference: ![diagram](graphs/foo.svg)

Actual Behavior

Because the generator injects <base href="./../../"> on the nested page, it alters the base URL for the entire document, breaking both rules defined above:

1. Markdown Images (Overshoots local directory)
The relative image ![diagram](graphs/foo.svg) is rendered in the HTML as <img src="graphs/foo.svg">.
Because of the <base> tag, the browser resolves this from the site root rather than the page directory.

• Expected: {site-root}/Verification/Key-Theorems/graphs/foo.svg
• Actual: {site-root}/graphs/foo.svg (404)

2. KaTeX Assets (Overshoots site root)
Absolute internal assets like /-verso-data/katex/katex.js are currently relativized by the compiler to ../../-verso-data/katex/katex.js.
Because the <base> tag already navigates the browser back to the site root, the additional ../../ prefix causes the browser to overshoot the root entirely.

• Expected: {site-root}/-verso-data/katex/katex.js
• Actual: {two-levels-above-site-root}/-verso-data/katex/katex.js (404)

Browser console output shows:
GET https://example.com/pages/-verso-data/katex/katex.js net::ERR_ABORTED 404

Proposed Fix

I have identified the root causes for both of these URL resolution issues in Html.lean and VersoManual.lean and opened a PR with a proposed fix here: #835

The PR adjusts the compiler to ensure user-authored relative paths are prefixed with their page path before the <base> tag shifts resolution, and prevents absolute asset URLs from generating redundant ../ sequences.