Is your concern that some browser may add the attribute *before* `checked`? I haven't done a full test of popular browsers, but my suspicion is that the attributes will always be appended to the attribute list, unless modification of a pre-existing attribute is taking place.

DOM Level 2 [doesn't explicitly instruct](http://www.w3.org/TR/DOM-Level-2-Core/core.html#ID-F68F082) the attribute to be *appended*.