Proposal: Optional MathJax Advanced Loader submodule (conditional loading, version selection)

Problem / Motivation

The current MathJax module loads the MathJax library globally on all pages where it is enabled,
regardless of whether mathematical notation is actually present on the page.

On educational, documentation, and publishing sites, this leads to unnecessary JavaScript payload,
avoidable performance degradation, and negative impact on Core Web Vitals (notably LCP and INP)
for pages that do not contain math content.

Additionally, site builders currently have no supported way to:

• Select between MathJax v2 and MathJax v3
• Conditionally load MathJax based on content type or route
• Opt into async or deferred loading strategies

Proposed solution

Introduce an optional submodule (working title: MathJax Advanced Loader)
that extends the existing MathJax module without changing its default behavior.

The submodule would provide opt-in configuration for:

• MathJax version selection:
  • MathJax v2 (legacy / default)
  • MathJax v3 (opt-in)
  • Custom CDN URL
• Conditional loading strategies (mutually exclusive):
  • Load only on selected content types
  • Load only on matching route patterns
  • Optional experimental auto-detection of math delimiters in rendered content
• Optional async / defer loading, explicitly enabled by administrators

All features would be disabled by default. Sites not enabling the submodule would experience
no behavior changes.

Scope and compatibility

• No changes to existing MathJax defaults
• No modification of rendered content
• No user-agent or crawler-specific behavior
• Compatible with render caching and Dynamic Page Cache

The implementation would attach MathJax libraries conditionally using
hook_page_attachments_alter(), following established Drupal patterns.

Next steps

If the approach is acceptable, I am happy to:

• Create a sandbox implementation
• Submit an initial patch for review
• Adjust scope based on maintainer feedback