Created attachment 278399 [details]
Page with css statement applied
Although the tab lockup of #148573 seems fixed in Safari 9, it seems that it is incorrectly measuring the width of inline elements (spanning multiple lines ???). Content overflows outside of the expected boundaries and random spacing is introduced between words.
Attached are a screenshot of a page with -webkit-isolate enabled on the .comment class, a screenshot with the rule disabled and a webarchive of the page.
This problem does not occur in Chrome.
Created attachment 278400 [details]
Page with css statement disabled
Created attachment 278401 [details]
webarchive of page in the screenshots
Created attachment 278517 [details]