Does it resolve the ability to use double lines in tables? Or does it enable the use of workarounds to make double lines everywhere? Because a workaround already exists, at least for other places. If you are using header without the need for its repetition, then it’s only for later accessibility (and styling). In this case semantically using header, but also semantically adding empty cells above it is a net-zero gain, IMO. If you really want to, you can just not use header and use the empty cell hack now. This is not the same as having a (built-in/non-hacky) way to create double lines.