I'm going to post a job on Upwork to create static HTML+CSS from Sketch designs. I want to be able to say in the job description: "must achieve FOO rating on BAR code quality test."

It's good to use automated metrics as a shared baseline for a gig like this, because the client and dev hardly know each other, and because that lets the dev know how they will be graded.

Can you recommend any metrics we could use?

Google Mobile Friendly is the kind of thing I'm thinking of. However I'd like something which applies to code quality.