needs to be XHTML, not HTML

[danielweck]

document.body.tagName ==> "BODY" (uppercase is a clear sign that the content type is incorrect)

Rename the file extension to .xhtml and serve with correct content-type HTTP header:

https://github.com/edrlab/w3c-annotation-selector-demo/blob/main/web/index.html

[panaC]

https://edrlab.github.io/w3c-annotation-selector-demo/web/index.xhtml