Defining elements

The elementRule and tag elements The rules for an element's content are expressed in elementRule elements, while rules for attributes are expressed in tag element. Every elementRule must have a tag:
```
<elementRule role="hr" label="hr">
  <empty />
</elementRule>
<tag name="hr" />
```
role and label are separate The label element is optional. If omitted, the label for the elementRule is the same as its role.
name matches role The tag element's name attribute value must match a role attribute of an elementRule, and it can also have its own role.