XML Base (Second Edition)
https://www.w3.org/TR/2009/REC-xmlbase-20090128/
> In particular, xml:base="" does not reset the base URI to that of the containing document.
xml:base、一度設定したが最後、もうリセットはできないのがなぁ。いや、わかるにはわかるんだが。
XML DOM での扱いとかどうなってるんだっけと思って。
Node: baseURI property - Web APIs | MDN
https://developer.mozilla.org/en-US/docs/Web/API/Node/baseURI
> its value is determined by an algorithm each time the property is accessed, and may change if the conditions changed.
それは助かる