Implement script element loading and execution #285

Implements issue 3mhyfp5om4p26 — Script element loading and execution (defer, async).

Changes #

New: script_loader module (crates/browser/src/script_loader.rs) #

• Walks the DOM in document order for <script> elements after HTML parsing
• Executes inline scripts and fetches external scripts via ResourceLoader
• All scripts share a single JS VM with full DOM access (attach_document)
• Respects type attribute: only executes text/javascript or no-type scripts
• defer external scripts are queued and executed after all synchronous scripts
• async scripts execute immediately (no ordering guarantee)
• Script errors are caught and logged without crashing the page
• After execution, document is detached from VM and returned for rendering

JS compiler: top-level var stores to global scope #

• Top-level var declarations now also emit StoreGlobal, matching function declaration behavior. This ensures variables persist across separate execute() calls when multiple scripts share a VM.

VM: detach_document() method #

• Reclaims the Document from the DOM bridge after script execution
• Enables the pattern: parse → attach → execute scripts → detach → render

Resource loader: Script variant #

• Added Resource::Script for JavaScript MIME types
• Updated MIME classifier with text/javascript, application/javascript
• Exhaustive match updates in css_loader and img_loader

Integration in main.rs #

• load_page() now calls execute_page_scripts() between HTML parsing and CSS/image collection, so scripts can modify the DOM before styling.

Tests #

9 new tests covering:

• Inline script execution with DOM modification
• Multiple scripts sharing global scope
• Unknown type attribute prevents execution
• type="text/javascript" executes correctly
• Script errors don't prevent subsequent scripts
• Empty scripts don't crash
• defer inline scripts (treated as sync per spec)
• createElement/appendChild from scripts
• Pages with no scripts pass through unchanged