No, syntax is not merely bikeshedding, but endless discussions about it can certainly turn into that. I don't know what qualifies as "mainstream", but anything that sprang from the head of Milner, you can be sure, put semantics first. As, for that matter, did work from Dijkstra, Hoare, etc. Lots of good semantics work out there that has often indirectly had a huge influence on the mainstream.