The main bulk of this stylesheet is an identity transformation so... XHTML elements are copied sans prefix (XHTML is default namespace here, so these elements will still be in XHTML namespace This just ensures the mathml prefix declaration isn't copied from the source at this stage, so that the system will use the mml prefix coming from this stylesheet namespace="mml" implementation="#mmlFactory" namespace="svg" implementation="#AdobeSVG" Somewhat bizarrely in an otherwise namespace aware system, Microsoft behaviours are defined to trigger off the prefix not the Namespace. In the code above we associated a MathML rendering behaviour (if one was found) with the prefix mml: so here we ensure that this is the prefix that actually gets used in the output.