The word-wrap:break-word property (being renamed to overflow-wrap in CSS3 Text) inappropriately causes breaks between a base character and its applied diacritics, and (even worse) breaks between the two code units of a surrogate pair.
<div style="width:0px;word-wrap:break-word">abc d̥e̕f̣
Argh, I forgot bugzilla would truncate my text as soon as it encountered a surrogate codepoint.
<div style="width:0px;word-wrap:break-word">abc d̥e̕f̣ %F0%90%90%80%F0%90%90%81%F0%90%90%82
Note the "blank" lines between the Deseret letters, because internally we split the low surrogate onto a separate line. (The fact that they're split across lines can be demonstrated by drag-selecting one of the Deseret glyphs (or hexboxes), *or* one of the invisible blanks in between them, and copy-pasting it into the search box - you get just the single, unpaired surrogate codepoint.)
Created attachment 621571 [details] [diff] [review]
patch, respect clusters when doing word-wrap:break-word
It looks like this can be trivially fixed, actually - we just need to check whether the current character is a cluster start when considering a possible breakpoint for word-wrap:break-word.
Created attachment 621572 [details] [diff] [review]
add a reftest for word-wrap cluster support