In fairness, this is HN's fault - not allowing posts to mess up the whole page is basic fit-for-purposeness for forum software and the whole reason HTML is not allowed in the first place. Your workaround, on the other hand, breaks the content as bookmarklets are always one-liners. Evidently (according to a sibling comment) the solution is to prepend 4 spaces, which isn't exactly standard.

As for the bookmarklet, smashing stuff. I'm always deleting irritating stuff on pages using the inspector.