If you have the time, just talk to them. Make sure they have some understanding of HTML and CSS. But HTML and CSS are the easiest to learn. Don't put down someone for not knowing the exact name for some semantic tag in HTML. If your project needs it, google will solve that problem in minutes. The problem is in Javascript. Make sure they understand OOP and make sure they understand prototypal inheritance. Also, if they can reason in a functional way and can deal with functional patterns, is a big plus.
You could make a test they should code on a white board to watch the reasoning, but do not focus on the details, but on the mindset. A task should be complex enough to test the understanding of the web stack, but simple enough to be doable. Finishing the task should not be the main concern, but the reasoning.
I would have them design an search input field with autocomplete. It looks easy, and if you know what you are doing, it is easy. But there are lots of interesting details one should know, in order to make it work. Things like interacting with the DOM, with an API, with the events... Lots of details in one should be careful about when designing such a thing.
This has value only if you do plan to have apps that use server side logic, but if you plan to build only flashy static websites, you could think of something else, like a drag and drop, a dynamic toolbar, or anything similar. Building any of those components in pure javascript is simple, but lots of people fail to do it.
Another thing you could do, is to have a requirement to have button with an ID that when clicked, will show another form, and there should be another button with the same ID, and have them reuse the ID's shown in the page. To use in real life, it is bad practice, but it shows if the person in front of you can deal with managing such a situation. The problem is not hard. Is just a bit strange. And make sure they understand the need to have private properties on objects and how to make objects with private properties.