Here are the steps a binding goes through when it is bound to a tag in a target document:
When loading the target document, the Mozilla layout engine detects the need for a binding when it sees a -moz-binding style on a given loaded tag.
The document's list of things to finish is incremented by one. This list delays the firing of an onload event for the target document.
If a complete, compiled copy of the binding does not yet exist for the target document, steps 4-7 are done. If it exists, processing continues from step 8.
The XBL document for the binding is fetched and put in the Mozilla cache. This is done in parallel with the loading of the target document.
The fetched XBL document is parsed and ...