Bug 1895923 - Fix browser_parsable.css. r=aleca

Differential Revision: https://phabricator.services.mozilla.com/D209935

--HG--
extra : moz-landing-system : lando

mail/test/static/browser_parsable_css.js

@@ -48,6 +48,11 @@ const ignoreList = [
     errorMessage: /Unknown property.*overflow-clip-box/i,
     isFromDevTools: false,
   },
+  {
+    sourceName: /\b(html)\.css$/i,
+    errorMessage: /Error in parsing value for ‘content’/i,
+    isFromDevTools: false,
+  },
   // These variables are declared somewhere else, and error when we load the
   // files directly. They're all marked intermittent because their appearance
   // in the error console seems to not be consistent.