And in Firefox, the text sometimes vanishes off the right-hand side of the screen. Not much - just a few mm, up to 1cm or so - but again, it's a nuisance.

This is usually caused by broken CSS making invalid assumptions about available fonts, screen resolution etc. Or, slightly more generally, by people trying to misuse HTML+CSS as a full-fledged page description language, rather than a markup language with formatting extensions bolted on.