In the attached test case, one slider has an oninput handler that changes the style of another element. Strangely, this has two side effects: 1) The thumb's initial position ignores the value attribute. 2) Moving the slider's thumb moves a second, unrelated, slider's thumb along with it.
Created attachment 14644 [details] Test case
I think this is caused by style sharing between the two sliders' thumbs.
Created attachment 14653 [details] Avoid sharing the thumb style
Comment on attachment 14653 [details] Avoid sharing the thumb style r=me
Landed in r21739