Bugzilla

FWIW, in case it's helpful, https://treeherder.mozilla.org/#/jobs?repo=autoland&group_state=expanded&selectedTaskRun=S_VZPZjoQM2arxf3psr85g.0&resultStatus=success%2Ctestfailed%2Cbusted%2Cexception&searchStr=os%2Cx%2C10.14%2Copt%2Cmochitests%2Ctest-macosx1014-64%2Fopt-mochitest-browser-chrome-e10s%2Cbc3&revision=e09fe2a6602b0c8d0b9ceb5f869456f01cefa445 shows that the failure is that line 511 in `checkAllTheCSS` fails. That line is https://hg.mozilla.org/integration/autoland/file/e09fe2a6602b0c8d0b9ceb5f869456f01cefa445/browser/base/content/test/static/browser_parsable_css.js#l511 :

```js
  doc.head.innerHTML = "";
```

which threw an NS_ERROR_FAILURE exception (which is confusing; using an empty string to clear out an element shouldn't even really be invoking any sanitizer code, I wouldn't have thought?).

It also failed on Linux and Windows and on opt builds, so it should be straightforward to reproduce locally on a non-debug build? I wouldn't be surprised if it also reproduced if you reduced the number of files it loads.

FWIW, in case it's helpful, https://treeherder.mozilla.org/#/jobs?repo=autoland&group_state=expanded&selectedTaskRun=S_VZPZjoQM2arxf3psr85g.0&resultStatus=success%2Ctestfailed%2Cbusted%2Cexception&searchStr=os%2Cx%2C10.14%2Copt%2Cmochitests%2Ctest-macosx1014-64%2Fopt-mochitest-browser-chrome-e10s%2Cbc3&revision=e09fe2a6602b0c8d0b9ceb5f869456f01cefa445 shows that the failure is that line 511 in `checkAllTheCSS` fails. That line is https://hg.mozilla.org/integration/autoland/file/e09fe2a6602b0c8d0b9ceb5f869456f01cefa445/browser/base/content/test/static/browser_parsable_css.js#l511 :

```js
  doc.head.innerHTML = "";
```

which threw an NS_ERROR_FAILURE exception (which is confusing; using an empty string to clear out an element shouldn't even really be invoking any sanitizer code, I wouldn't have thought?).

Edit: Oh, and this is the content document of an iframe which we remove from the DOM on the previous line.

It also failed on Linux and Windows and on opt builds, so it should be straightforward to reproduce locally on a non-debug build? I wouldn't be surprised if it also reproduced if you reduced the number of files it loads.