Safari and Firefox both have built-in default behavior that's sort of like text-decoration-skip-spaces: end, but not quite the same as the one specified, and not quite the same as each other:

• Safari skips all preserved U+0020 (but not  , or U+3000, or U+2007) on the end side if white-space is pre-wrap, but not pre or break-spaces
• Firefox skips overflowing preserved U+0020 (but not  , or U+3000, or U+2007) on the end side if white-space is pre-wrap, but not pre or break-spaces

I wonder if we need to make the end value (and maybe other values) smarter, or if we need an auto of some kind in addition, or if this is just historical accident and all UAs will align on the specified behavior of end

Also, the spec says that the initial value of text-decoration-skip-spaces is start end, which is consistent with what level 3 said in prose. However, Chrome behaves as if the initial value was none, and Firefox / Safari behave as if the initial value was magic-end (see previous point) and don't skip leading spaces at all. Not sure if the spec should align with the browsers or the other way around, but the discrepancy is worth noting. We do have a resolution in favor of what the spec says, so it's probably just a matter of writing tests and filing bugs…