In the URL, there are two-pixel long black lines right above the "Standalone Installer" and "USB" headings. They are apparently underlines painted under <br>s. Highlighting nearby text deletes one of the pixels, and forcing a repaint restores it.
The underline painting under <br> seems to be a feature of strict mode. This is a reduction: <!DOCTYPE HTML><u><br>foo</u>
Created attachment 15147 [details] Do not paint zero-width underlines, paint short underlines properly
Comment on attachment 15147 [details] Do not paint zero-width underlines, paint short underlines properly r=me
Landed in r23761.