This is an interesting issue if not bug. I have copied the Standard page template (version 22.1.7) within the universal theme to a separate template and made a small modification in the footer area. The line I changed was this…
<a href="#top" id="t_Footer_topButton"><span><img src="#APP_FILES#/hc-up-chevron.png" alt="Start of Page"></span></a> (I added a chevron with some alt text). You can find this at the bottom of the body section… at least the reference to #top.
That said, when I run the page I see “Start of Page” when my mouse goes over the image. However, when I inspect the page source… the line appears as this…
<a href="#top" id="t_Footer_topButton" title="Start of page"><span><img src="r/portal/500/files/static/v102//hc-up-chevron.png" alt="Start of Page"></span></a>
Now if you notice, title="Start of page" has magically jumped into this line when I view the page source through the browser. From where did it come? It's not in the template file above at all. Normally I would not care, but we support english and french. So while I can update the alt text for the chevron and it shows in french, the title cannot be translated because it does not show up in the xliff file because its not in the template to begin with. Thus, you cannot translate it and it is what is being shown when your mouse goes over the image… not the alt text. We still want to be able to go to the top of the page so removing the line is not an option. It is like apex is introducing the title arbitrarily for whatever reason based on the #top designation. It would be ok if it was translatable but it's not showing in the template.
So I'm not sure how to fix this, because it doesn't show up in the template in the first place. This would appear to me to be a bug of some sort.