Stylesheet Loading

Stylesheet Loading

OPBrowser now loads initial external author stylesheets from <link rel="stylesheet"> without delegating CSS parsing or cascade behavior to another browser engine.

Supported sources

Active stylesheet links may load from:

• relative or absolute local paths when the document itself is local;
• file: URLs under the same local-source policy;
• http: and https: URLs through the existing WinHTTP transport;
• data:text/css URLs.

A network document cannot use a stylesheet link to escape into local files. An HTTPS document cannot downgrade a stylesheet request to HTTP.

Link activation subset

A <link> participates when:

• rel contains the ASCII-case-insensitive stylesheet token;
• rel does not contain alternate;
• disabled is absent;
• type is absent, empty, or text/css;
• media is absent, empty, all, or screen;
• href exists and resolves under the source policy.

Full media-query evaluation is not implemented yet. print and other media values are currently skipped instead of guessed.

Loading and budgets

External CSS is discovered during page preparation on the same navigation worker that loads the document and image subresources.

Current bounds per prepared document are:

• at most 32 stylesheet link nodes are considered;
• at most 8 distinct stylesheet requests are attempted;
• one stylesheet is capped at 1 MiB;
• total decoded stylesheet text is capped at 2 MiB;
• stylesheet discovery/loading has a 10 second overall guard;
• HTTP stylesheet requests use a 2 second WinHTTP timeout and 5 second read deadline.

Duplicate resolved stylesheet URLs share one fetched text payload but each link node keeps its own position in document source order. The cache also preserves the final stylesheet address after redirects; each link NodeId retains that effective base in PreparedDocument. Engine exposes active_stylesheet_address() for resource-base inspection. Reflow reuses this metadata without fetching again.

A stylesheet load failure is nonfatal to the HTML document. The failed link contributes no rules and preparation continues.

HTTP and decoding

HTTP requests send an Accept preference for text/css. An explicit response MIME type other than text/css is rejected; a missing Content-Type is temporarily accepted.

CSS text decoding uses this initial priority:

1. Unicode BOM;
2. HTTP/Data URL charset parameter;
3. an initial exact @charset "..."; rule;
4. UTF-8 default.

The currently owned decoder supports UTF-8, UTF-16, Windows-1251 and Windows-1252. The consumed initial @charset rule is removed before the CSS parser sees the text.

Cascade order

Loaded external stylesheets are attached to their DOM link NodeId. op_css traverses the document and parses embedded <style> and loaded <link rel="stylesheet"> rules at their actual DOM positions. This preserves stylesheet source order across external and embedded rules before inline style="" declarations enter the author cascade. MatchedDeclaration.style_node identifies the originating style/link node or inline-styled element. Computed var() copies preserve that provenance for later CSS resource resolution.

A successfully prepared document retains its author StyleCollection and ComputedStyleMap. Resize reflow therefore does not refetch external CSS or rerun stylesheet discovery.

Deliberate current limits

This milestone does not implement:

• @import resource loading;
• general media queries;
• <base>-element URL changes for stylesheet resolution;
• CSS url(...) subresources such as fonts/background images;
• preload/module/style-set behavior;
• CORS/CSP/referrer/integrity processing;
• browser cache persistence across document preparations.

The parser currently reports unsupported at-rules rather than pretending they worked.

Verification

The local demonstration uses a real external file:

target\release\op_browser.exe examples\css\index.html

examples/css/index.html links examples/css/theme.css and also contains a later embedded rule to verify source-order override behavior.

Tests cover local external CSS, a missing nonfatal stylesheet, retained CSS after the source files are deleted and reflowed, loopback HTTP CSS requests, CSS MIME/charset handling, byte limits and HTTPS/local source policy.