[umvi]

Indeed, but this is a case where you are reducing lines of source code in exchange for (slightly) increased coupling and fragility. By replacing remove/add with replace, you have now made an assumption that causes the replace line to be coupled to the html lang line where no coupling existed previously...

[playpause]

It's a well known convention (CTM), it's been part of a lot of front end application shells for years. Like other shells, an HTML boilerplate is somewhat expected to be a delicate, hand-optimised system. When it needs revising (which should be very rare) it should always be reasoned about as a whole. Its job is to bootstrap the front-end environment with maximum efficiency, and that means doing things that would be considered bad programming in another context.