Issue description:

To Reproduce visit https://developers.google.com/fonts/docs/getting_started#enabling_font_effects_beta in Google Chrome or similar browser.

Expected behaviour: masked text samples visible / readable, like currently visible in archived snapshot

(Additional context:) I've posted this problem as a snark illustration of what I see as a specification flaw (failed mask response makes whole target element invisible) in w3c/fxtf-drafts#130 (comment)

Good move here would obviously be to move mask image files next to font files to avoid server configuration mismatch (and possible security firewalls blocking of googleusercontent domains). Or retiring/ending this beta experiment altogether.

Oh, or adding CORS headers for themes.googleusercontent.com responses is also an option.