What qualities should be considered when designing a front end project? I'm working on designing a project where the students will use HTML+CSS to replicate a specific layout which I will give them as a picture. The goal of the project is to allow each student to practice their HTML+CSS skills by implementing the design in a clean way that follows the best practices that I have taught.

I would want to give each person a different task (or at least have more than one design so that each person could get a random design to implement). In order to ensure that the assignments I design are challenging, interesting, distinct, and of similar difficulty, what qualities should I try to emphasize? Keep in mind that these students should have a full knowledge of HTML+CSS, so anything can be included.

Both specific ideas for layouts and CSS/HTML skills to try to include are valuable. Some things that I have thought of have included multi-column layouts, a sticky nav, or notifications/lightboxes.
As a student I would find this assignment uninteresting and a waste of my skills. I would feel the professor is just trying to be lazy and not having to properly grade assignments.

If they really should know HTML and CSS by now, don't treat them like babies. Just ask them to build a complex responsive page that works as their resume and/or portfolio. They will be glad to make something beautiful and actually interested in your assignment. You can of course set minimum requirements like having transitions and animations, media queries, etc.

My point is: don't create an assignment to try to make your students fail a few points off your check list, but create one that will make all your students interested and proud of their creation.