Reproduce steps: 0. Set 'dom.webcomponents.enabled' to true. 1. Save 'blue-text.css' and 'blue-text.htm' into a folder. 2. Open 'blue-text.htm'. Two black lines appear. But they should always be blue. 3. Open Inspector in DevTools. Inspect the 'body' element. All lines turn blue now. 4. Refresh the page. All lines turn black again. 5. Refresh several times more. The second line turns blue sometimes, but the first line always be black.