The edited files are typically small enough -less than a few megabytes- to easily fit in memory (and my desktop has 64Gbytes)
A typical application could be useful in a shell script which replaces, in the hand-written HTML file of my draft guile/scheme mini tutorial for Linux developers, the
<span class='date'>june 30, 2019</span> line by a line of the same
<span class='date' holding the current date and time (e.g. as obtained with
date +%c). I feel that XSLT is overkill for such a simple thing.