Currently `float: inline-start/inline-end` is only supported in Firefox, see https://developer.mozilla.org/en-US/docs/Web/CSS/float#browser_compatibility, and in order to support other browsers we're thus forced to jump through some hoops.
This leads to slightly less nice code in the *built-in* Firefox PDF Viewer, and this patch attempts to improve the current situation:
- Use Stylelint to forbid direct use of `float: inline-start/inline-end` in the CSS files, to prevent future bugs in the general PDF.js viewer.
- Do a build-time replacement, only in MOZCENTRAL builds, to replace the CSS-variables with raw `float: inline-start/inline-end` instances.
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