CSS implementation of <ms> does not work in RTL mode

[fred-wang]

cc @emilio

(See also #120 about whether ms is actually useful / important or if we could just remove it from core)

The CSS implementation of MathML Core uses the pseudo elements :before and :after to implement the ms element: https://mathml-refresh.github.io/mathml-core/#string-literal-ms

Not sure whether it's described explicitly in CSS, but the quotes are not inverted in RTL mode, contrary to the MathML3 expectation. The following test exported from Gecko's test suite fails in all browsers: https://w3c-test.org/mathml/presentation-markup/direction/direction-009.html

It has been failing in Gecko after some refactoring to move to a CSS-based implementation and avoid some security/design issues. For details, see https://bugzilla.mozilla.org/show_bug.cgi?id=787215

[fred-wang]

Pure HTML/CSS test:

<style>
  .ms[lquote]:before {
      content: attr(lquote);
  }
  .ms[rquote]:after {
      content: attr(rquote);
  }
</style>
<div style="width: 15em">
  <div>
    <div class="ms" lquote="START" rquote="END">[LTR]</div>
  </div>
  <div>
    <div class="ms" style="direction: rtl;" lquote="START" rquote="END">[RTL]</div>
  </div>
</div>

of using valid HTML code:

<style>
  div {
      quotes: "START" "END";
  }
  .ms:before {
      content: open-quote;
  }
  .ms:after {
      content: close-quote;
  }
</style>
<div style="width: 15em">
  <div>
    <div class="ms">[LTR]</div>
  </div>
  <div>
    <div class="ms" style="direction: rtl;">[RTL]</div>
  </div>
</div>

[fred-wang]

Consensus from 2019/11/11: Keep ms in MathML Core but make it behave like mn and add a note to tell people to either put the quotes in the contents or add CSS to style it via before/after.

(we didn't agree put probably the note should mention this bug in RTL mode)

[fred-wang]

merged into #120