Passing context changes base font-size on Internet Explorer

[yuderekyu]

Description

If a context is passed to axe.run(), the accessibility check alters the root font size.

I inject axe into an Internet Explorer 11 driver instance (selenium grid 2.53.0). My component has a font-size set to 1.714rem and the base font size is 14px. This translates to a font size of 24px. But, accessibility returns an error stating that the font size is 17.9px, which then fails color contrast ratios for non large fonts. If I do not pass in a context and default to document, there are no errors.

Chrome and Firefox do not cause issues. I have updated axe-core to the latest release, but the issue still persists.

Expectation: axe.run to not alter base font-size based on context for supported browsers.

Actual: axe.runalters base font-size based on context on internet explorer

axe-core version: 3.0.3

[WilcoFiers]

To the best of my knowledge, axe-core never sets any CSS properties. We'll have to investigate. Do you have a way for us to reproduce the issue? That would help narrow it down.

[straker]

Confirmed that when passing a context into axe.run, IE11 throws a color-contrast issue. https://codepen.io/straker/pen/Ezzxbw.

The issue seems to be that in IE11, the final computed font-size of the element is 23.93px or 17.9pt, which makes the element fall under the normal text size contrast ratio. I'll dig into why without a context it accepts it but with one it doesn't.

[straker]

Found the bug. In IE11, document does not have the msMatchesSelector function in element-matches.js so color-contrast errors out when it tries to get all elements on the page starting at document. If you look at the results for a no-context run, the incomplete section shows the error (not helpful, but it's there).

It gets the document because in findUpVirtual the closest function doesn't exist so it scans up the tree using parentNode, which is document for the htmlElement

[yuderekyu]

Awesome @straker! I see now I misread pt as px. Looks like IE rounds font-size to the hundredth spot https://phabricator.wikimedia.org/T102364

@WilcoFiers I'm sorry I didn't produce a reduce test case in time to speed up this process

[padmavemulapati]

Verified the fix availability, Done unit testing. Results are good , no failures.