[css-ui-3][svg] Is SVGZ support actually mandatory?

[frivoal]

The following test in the css-ui-3 test suite checks if gziped svg files are supported as cursors:

https://github.com/w3c/csswg-test/blob/master/css-ui-3/cursor-image-007.html

I cannot find a reference requiring browsers to support SVGZ. CSS-UI does require SVG support, but unless the SVG spec says that this must include SVGZ support, then there's not justification for this test being there as a MUST pass.

I'm happy to keep it with a may flag since additional file formats are allowed, but I don't think we should block CSS-UI progress on lack of support for this format that seems to actually be optional.

@svgeesus Am I missing a MUST requirement somewhere for SVGZ to be supported?

[frivoal]

Note: this is one of the few tests that don't have 2 implementations in the test suite.

[frivoal]

Alternatively, the bug may be with the test-suite / server, if .svgz files are expected to be served with a Content-Encoding: gzip http header, since that is currently not the case.

[frivoal]

As far as I can tell from https://www.w3.org/TR/SVG11/conform.html#ConformingSVGViewers
UAs are indeed not supposed to open the file if the Content-Encoding header is wrong:

When an SVG viewer retrieves compressed content (e.g., an .svgz file) over HTTP, if the "Content-Encoding" and "Transfer-Encoding" response headers are missing or specify a value that does not match the compression method that has been applied to the content, then the SVG viewer must not render the content and must treat the document as being in error.

I'll make a PR to fix that.