Summary: A way to feature-detect which values of various tokenlist
attributes actually do something. Useful for things like which iframe
sandbox tokens are supported, or what "rel" values for <link> are supported.
Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1257849
Link to Standard: If only it were so simple. The basic feature is
defined at https://dom.spec.whatwg.org/#dom-domtokenlist-supports but it
depends on consumers of token lists defining the set of "supported
tokens" for them. In practice that means HTML. HTML defines supported
tokens for "rel" on <a>, <area>, and <link>, "sandbox" on <iframe>, and
"dropzone" on all elements. Most of these definitions are sane, but the
<link> one includes a "yeah, just monkeypatch this" aspect that makes it
a bit hard to figure out what the actual set of supported tokens is.
Anyway, the relevant definitions are:
https://html.spec.whatwg.org/multipage/semantics.html#attr-hyperlink-rel
https://html.spec.whatwg.org/multipage/embedded-content.html#attr-iframe-sandbox
https://html.spec.whatwg.org/multipage/interaction.html#the-dropzone-attribute
https://html.spec.whatwg.org/multipage/semantics.html#linkTypes
In practice, this is the set of <link rel> values I'm claiming we
support: "import" (if IsImportEnabled()), "prefetch", "dns-prefetch",
"stylesheet", "next", "alternate", "preconnect", "icon". Anything else
I'm missing? Do we do anything useful with rel="feed" anymore?
Estimated or target release: Firefox 49
Preference behind which this is implemented: none.
DevToolks bug: none; I'm not sure we need anything special here.
Support in other browsers: Blink supports this, though only for
iframe@sandbox and link@rel (because its a@rel is not a DOMTokenList).
Not sure about others.
Security/privacy concerns: none.
-Boris
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