How does this support UDL?

Since the content is both free to access and utililzes both video/audio content for teaching and space for practice of the concepts taught, it allows for anyone with a basic understanding of the internet (navigation of websites, mouse handing abilities, etc.) to have access to the videos. As well, since there is an interactive transcript on many of the videos, it allows students with hearing impairments to follow along with what the video is teaching.

Are there any requirements to sign up?

You can access most, if not all of the content on the website without an account, but if you would like to track your progress, you do need a free account. The only restriction on the account is that you haven't been removed from the website for harassment prior to your signup, and that you are either 13 years or older, or have consent from a parent or legal guardian.