Fedor2, thanks for Your attention.
To trick some settings in the "about:config" of Pale Moon - for to switch off the exclamation mark of the "Youtube/html5" 6 supported things -, can not be the way, so my last thoughts to that.
After having installed - with the help and advice from Lyceus - the last K-Lite Codec Pack, I think, the Blue Screens crashes of some (big company) sites with html5-technics have disappeared.
But this I must test once conscious and intentional. (Tested it with one site and it seems to be OK now this.)
There are so many confusing things represented if reading things about html5. It depends on where Your read, depending on from whom, with which interessts and intentions.
After thinking and getting clear, for me, it is not the point, not THE MAIN, to have this html5 WORKING, I just DO NOT WANT Blue Screens crashes, first.
I am happy with working videos from Youtube as flash or what ever, if only the video is to view and the sound
This already - with Youtube - worked even with my older K-Lite Codec Pack, or maybe more, I think, the Flash-player plug-in in Pale Moon.
So, after thinking about it, I do not need the html5 working at all, where the Flash-player DOES it.
The ´only´ problem is - but will get probably a problem with time, with this development from some interessts of propagating this html5 this way -, where only a html5-player plays THIS video still, with no alternative.
For example this link https://www.youtube.com/watch?v=NuIAYHVeFYs
(Title: Beautiful Relaxing Music 24/7: Study Music, Sleep Music, Meditation Music, Sleeping Music).
To this the newest K-Lite Codec Pack still does not help (for Pale Moon 26.5).
Also in my case of slow connection - if limit of 1 GB is used, after some days - of then still 128 KByte/s highest, I need LOWest data-sizes.
A german TV zdf.info - live-stream (only in Germany) and from mediathek - does it very well, one of less, I think, but this I do not know, but there is to see, it WORKs, even with 128 KByte/s.
And still this point please, from the talks here:
for Firefox there exist many, many html5-player plug-ins.
That it is not possible to create one - if Open Source or not - for Pale Moon, I cannot really imagine.
Fedor2, thanks for Your offer. I will subscribe at Your site and ask there.
I want to begin with installing Your Pale Moon XP - sadnessly Moonchild does not want exact this, to avoid to be brought together with this version, despite the user NEED to know, and as me, want to have it as name for to recognize it at the name and not first from having have to remind, oh, the Fedor2´s MYPAL is THE version of Pale Moon FOR XP -, parallel to the now working 26.5 version.
(And the common respect to Moonchild and his work . . ., but this is what he does not want, too. This makes it very hard, if a note to this at Your site is not enough for Moonchild, that this Your version is not from Moonchild (Productions), and support questions to things from this Your version are not welcome by Moonchild and by the Pale Moon Forum. To may name it Pale Moon XP (ONLY), or similar.)