See mathiasbynens/math-tex#2 (comment):
Why does
@font-face
work as expected on https://mathiasbynens.github.io/html-imports-font-face/standalone.html but not on https://mathiasbynens.github.io/html-imports-font-face/example.html?
@font-face
doesn’t seem to work from within HTML imports, but I don’t know why. Does any spec mandate this, or is this an implementation bug that just happens to be present in both Chromium and Gecko? Or am I doing something very wrong here?