It looks like Thunderbird is expecting a meta tag indicating the content-type of the markup. Here is the relevant Chromium bug: https://code.google.com/p/chromium/issues/detail?id=28888. I have a fix for this, but it breaks a bunch of tests that do not expect the meta tag in the markup. We may just have to check in a bunch of platform-specific results that say "FAIL" in them, unless I can figure out a way to not break the tests. :(
Created attachment 71831 [details] Patch for this issue which does not affect layout tests
Comment on attachment 71831 [details] Patch for this issue which does not affect layout tests Ugh :) OK, I guess.
Comment on attachment 71831 [details] Patch for this issue which does not affect layout tests Clearing flags on attachment: 71831 Committed r71031: <http://trac.webkit.org/changeset/71031>
All reviewed patches have been landed. Closing bug.