Define how requiredExtensions values should be interpretted

[AmeliaBR]

The value of requiredExtensions is a list of URLs. The user agent must support all those named extensions for the element to pass the conditional processing test. However, any details of what those URLs should be or how support should be assessed is left up to the browser and/or extension author.

Firefox supports the XML namespaces for MathML and XHTML as recognized extensions, but this isn't standard behavior defined anywhere.

It would be useful to define a set of standard URLs for common content that might be included as XML in a <foreignObject>, as an embedded document in an <iframe>, or as an image format. By explicitly supporting image formats as an "extension", the <switch> element can match the image format fallback behavior of the HTML <picture> element.

Alternatively, the content model for the attribute could be extended so that it allows other strings, such as a way to define MIME types. Maybe as a pseudo-protocol, that can be parsed like a URL? E.g. requiredExtensions="mime:image/webp".

[AmeliaBR]

On 2 June 2016 telcon, resolved to address this in the SVG Integration spec.