Auto merge of #6543 - Ms2ger:a-href-activate, r=jdm

Only make a elements activatable when they have an href attribute.

I've tested this manually, by clicking on the "baz" in code like

```js
var a = document.body.appendChild(document.createElement("a"));
a.textContent = "bar ";
a.setAttribute("href", "http://www.yahoo.com");
var b = a.appendChild(document.createElement("a"));
b.textContent = "baz";
```

but I've not found a way to write an automated test.

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