Roger b Do you need to know from XSLT, or is it sufficient to just know? I
use xmblwfb on the commandline almost daily. It is nearly instantaneous.
Here it processes all 830 TEI specification files (relatively small files,
that is) in < 0.04 s of real time:[1]

$ ls -1 *.xml | wc
    830     830   12112
$ time xmlwf *.xml

real 0m0.038s
user 0m0.027s
sys 0m0.011s
$

Here I deliberately introduce an ill-formedness and try again (just to prove
it is not a no-op p):

$ echo "</duck>" >> xenoData.xml
$ time xmlwf *.xml
xenoData.xml:275:1: not well-formed (invalid token)

real 0m0.034s
user 0m0.034s
sys 0m0.000s
$

Notes
[1] That is literally less than a heartbeat, almost no matter how you define
one. Just the electrical signal for ventricular contraction alone in healthy
subjects is > 0.06 s, let alone atrial contraction or the time in between. (In
an unhealthy heart ventricular contraction may take over twice that long).
[2] I am not sure, but I think xmlwf is part of the expat package in Debian,
and probably other distros.
________________________________