PDFjs' CSS sets `font-size: 10px` on the `html` element. This sets
1rem = `10px` which is tiny. Our styles work in rem units for spacing,
so padding, margins, etc. are too tight with that `10px` base size.

Set `html` font-size back to 100% for now. This does not have any
apparent effect on PDFjs styling, as `body` sets `font-size: 16px`
anyway.

See https://github.com/mozilla/pdf.js/issues/14555