Certain css properties not supported on all browsers

[maxgebs]

Bug description

Squares appear for icons that have the CSS properties mask-image, mask-repeat and mask-position, and those aren't supported on around 8% of browsers: https://caniuse.com/css-masks

How to reproduce

1. Try a gitbook site out one of the non supported browsers

See the icons here on the dropdown chevron and the search icon:

Additional context