Right now, TextExtracts:

- Strips the fallback images from the output of the Math extension; and
- Flattens the
`<span style="display: none;">`container element that wraps the`math`element and its children.

The result is that HTML and plain text extracts contain the alt text of the MathML, e.g.

https://en.wikipedia.org/w/api.php?format=jsonfm&action=query&prop=extracts&titles=Planck%20constant

and

"Ко́мпле́ксные чи́сла — числа вида {\displaystyle x+iy} x+iy, где {\displaystyle x} x и {\displaystyle y} y — вещественные числа, {\displaystyle i} i"

### AC

`math`elements (and their children) are stripped.`img.mwe-math-fallback-image`and`img.mwe-math-fallback-image-inline`elements aren't stripped from HTML extracts.- Whitelist should be configurable

- Plain text extracts behave as before, i.e. the alt-text of the MathML markup is still renderered in plain text extracts.
- Audio and video tags (other media tags) are stripped as before.
- Announce the change on the mediawiki-api-announce mailing list.

### Testing criteria

- Go to: https://en.wikipedia.beta.wmflabs.org/wiki/Lie_group
- Hover over lie algebra

Expected behavior: lie algebra preview should display all mathematical expressions across browsers