The original XSLT 1.0 rule for union patterns such as match="A|B" said that the default priority was calculated as if there were two separate template rules with match="A" and match="B". This became more complicated with the introduction of xsl:next-match in XSLT 2.0 - what should happen if the item matches both branches? It became more complicated again in XSLT 3.0 with the introduction of on-multiple-match - is it a multiple match if an item matches both branches? And in 4.0 it's complicated further by the introduction of constructs like match="element(A|B)" which is deemed equivalent to match=A|B.

I would like to break this cycle with a backwards-incompatible change. The default priority of a union pattern should be the numeric maximum of the default priorities of its branches; the treatment as being somewhat-equivalent to two separate template rules should go. We should encourage implementations to issue a compatibility warning if a union pattern appears with no explicit priority, and with multiple branches having different default priority.