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In the text added in rev 4803, the step "Set node to the previous entry in the stack of open elements." is not a sufficient advancement to make the loop terminate when node becomes a node that has already been popped by the secondary insertion mode in the previous iteration. To make things advance, it is necessary to jump right back to the "Loop:" label in that case. Or more importantly, you might want to abort processing the end tag if insertion mode is no longer "in foreign".
Adjusting importance per discussion with Hixie.
http://hg.mozilla.org/mozilla-central/file/62c09fac599a/parser/html/javasrc/TreeBuilder.java#l3109 shows how I improvised around this bug.
http://hg.mozilla.org/projects/htmlparser/rev/b08b9993cd5d#l1.64 is the fix for this bug.
EDITOR'S RESPONSE: This is an Editor's Response to your comment. If you are satisfied with this response, please change the state of this bug to CLOSED. If you have additional information and would like the editor to reconsider, please reopen this bug. If you would like to escalate the issue to the full HTML Working Group, please add the TrackerRequest keyword to this bug, and suggest title and text for the tracker issue; or you may create a tracker issue yourself, if you are able to do so. For more details, see this document: http://dev.w3.org/html5/decision-policy/decision-policy.html Status: Partially Accepted Change Description: see diff given below Rationale: If I understand this correctly, it is fixed by the fix to bug 9580.