Don't inherit SVG style properties from the environment.

[davidflanagan]

We don't want to inherit styles that affect SVG painting unless those
styles also affect regular text rendering. This patch adds styles
to the stylesheet so that every svg element in KaTeX output starts
off with a known set of basic fill and stroke attributes.

This should resolve issue #1088

[davidflanagan]

This screenshot was created by using the npm start testbed, and adding some new style properties on the

element that contains the KaTeX output. It demonstrates that with this patch applied, we inherit color, font-size and transform, but we are not affected by things like fill-opacity and stroke-width.

Compare this screenshot to the one in issue #1088

Note also that previously we inherited fill color for elements, but not the stroke color, so it was possible for the environment to add a colored outline to our svg paths. Now we inherit both fill and stroke on the svg element itself, so setting the stroke style does not give a colored outline.

[davidflanagan]

Okay, still need to make tests pass, but I'm hoping that the fix can be basically as simple as this.

[kevinbarabash]

@davidflanagan maybe add a screenshot test for this. It should be possible using the pre and post properties in ss_data.yaml to construct a container with styles that would affect the rendering without your changes. Something like:

SvgReset:
    pre: <div style="fill-opacity: 0.5; stroke-width: 3, ...">
    tex: \sqrt{x+1}
    post: </div>

[davidflanagan]

That's a great idea, Kevin. I'll do it.

[davidflanagan]

Actually, the pre and post attributes on the screenshot tests set the content of spans before and after the math, but they're both siblings of the math element, not ancestors, so I can't use them to set styles that would affect the math...

[kevinbarabash]

LGTM