Experimental support for web components. This field stores a
reference to the component implementation. It was inspired by Mozilla's
x-tags project. Please note: in the future it may be possible to
extend Element from your class, in which case this field will be
deprecated.
Parses the HTML fragment and sets it as the contents of this element.
This ensures that the generated content follows the sanitization rules
specified by the validator or treeSanitizer.