Conditional rendering via XML transform

[DrSensor]

Certain WG use xml transform like <xsl:if> for declarative custom element (DCE) 🤯

https://github.com/EPA-WG/custom-element

I highly recommend to check their code. However, their current implementation cause double parsing:

1. parse HTML (before custom-element:defined)
2. parse XML (after custom-element:defined)

The worst part is it parse the whole content inside \<custom-element>. So phase 1 is HTML->DOM then phase 2 is DOM->XML->DOM.

I didn't expect this since I try hard to avoid XML related API like XPath query.