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Currently polyglot says yes. But the issue depends on bug 23368.
It looks like the status is that this is currentlyh not permitted in HTML5 and that one doesn't want to consider it for HTMl5 but may be for HTMl5.1. From Polyglot Markup’s point of view, I think we should look at the issue to determine what is most polyglot and most robust. And I think that it is a *slightly* more robust/interoparable if xlink: is declared inside each <svg element rather than on the <html element simply because, if you copy the <svg> element but the xlink: is declared on the <html, then, unless it is declared on the root also in the document where you paste it in, the xlink: attributes will not work in application/xml+svg or application/xml+xhtml until you add the xlink: prefix declaration. So it is even thinkable that we should make it a polyglot markup principle that prefixes must be declared -to put it simply- in the context where they matter. But Eliot, while we think about those questions, I suggest to 1) remove the 'depends on bug 23368' link 2) remove the sentence that describes this as permitted. OK?
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: Accepted Change Description: Removed permission to declare on <html> and added description of where and why to declare it. Rationale: HTML5 says that only the XHTML namespace can be declared no html elements. Checked in a fix this revision: http://dev.w3.org/cvsweb/html5/html-polyglot/html-polyglot.html?rev=1.4 and http://dev.w3.org/cvsweb/html5/html-polyglot/html-polyglot.html?rev=1.5