We should avoid loading the html.css UA style sheet when possible. Currently we load it for _all_ documents because ua.css (which is loaded for all documents) @import's it. This means that we load it for XUL documents, SVG documents, SVG-as-an-image, etc.

(In reply to Jonathan Watt [:jwatt] from comment #0) > We should avoid loading the html.css UA style sheet when possible. Currently > we load it for _all_ documents because ua.css (which is loaded for all > documents) @import's it. This means that we load it for XUL documents, SVG > documents, SVG-as-an-image, etc. But these other types of documents can all include elements in the HTML namespace, and often do. So I don't think we can do this.

The patch (yet to be worked through Try) loads html.css on demand in nsGenericHTMLElement::BindToTree. Try will hopefully give me a better idea of how common it is for XUL to include HTML elements. If it's common it may be better to load html.css up front for XUL too. I don't think it's common enough for SVG documents.