Ensure element's namespace is HTML, SVG or MathML before covering eve… (#578)

* Ensure element's namespace is HTML, SVG or MathML before covering event handler content attributes

Author: lukewarlow

spec/index.bs

@@ -132,9 +132,18 @@ spec: HTML; urlPrefix: https://html.spec.whatwg.org/
     type: dfn; text: get the text steps; url: get-the-text-steps
     type: dfn; text: set the inner text steps; url: set-the-inner-text-steps
     type: dfn; text: src; url: attr-script-src
+    type: dfn; text: event handler content attribute; url: event-handler-content-attributes
 spec:DOM; urlPrefix: https://dom.spec.whatwg.org/
     type: dfn; text: get text content; url: get-text-content
     type: dfn; text: set text content; url: set-text-content
+    type: dfn; text: element namespace; url: concept-element-namespace
+spec:infra; urlPrefix: https://infra.spec.whatwg.org/
+    type: dfn; text: HTML namespace; url: html-namespace
+    type: dfn; text: SVG namespace; url: svg-namespace
+    type: dfn; text: MathML namespace; url: mathml-namespace
+    type: dfn; text: XLink namespace; url: xlink-namespace
+    type: dfn; text: list; url: list
+    type: dfn; text: list contain; url: list-contain
 </pre>
 
 <pre class="link-defaults">
@@ -1111,7 +1120,7 @@ To <dfn abstract-op>Get Trusted Type data for attribute</dfn> given |element|, |
 Issue: The [=event handler content attribute=] concept used below is ambiguous. This spec needs a better mechanism to identify event handler attributes. See [https://github.com/w3c/trusted-types/issues/520](https://github.com/w3c/trusted-types/issues/520).
 
 1. Let |data| be null.
-1. If |attributeNs| is null, and |attribute| is the name of an [=event handler content attribute=], then:
+1. If |attributeNs| is null, « [=HTML namespace=], [=SVG namespace=], [=MathML namespace=] » [=list contain|contains=] |element|'s [=element namespace|namespace=], and |attribute| is the name of an [=event handler content attribute=]:
     1. Return ({{Element}}, null, |attribute|, {{TrustedScript}}, "Element " + |attribute|).
 1. Find the row in the following table, where |element| is in the first column, |attributeNs| is in the second column,
     and |attribute| is in the third column.