Fix startup-css-check by updating utility used

See this comment for more context.

Startup CSS is a duplication of only the critical CSS needed on initial page load. It reads from HTML fixtures to determine what CSS is actually used.

There's a small discrepancy between the HTML in jh/.../layout/devise.html and the one in CE. This MR resolves that discrepancy as a quick fix for the startup-css-check job.