you can do blur with css have a look at the backdrop-filter thing as well

but I think the bigger problem here is getting the content to be behind the titlebar, because the titlebar and the content share a common parent and you'd have to position the titlebar to be above the content, probably with negative margins?

then you have to worry about the page being translated up, so you'd somehow have to modify all pages to have a top margin for their body or something so it doesn't look off