If you access any GitHub Pages websites at <username>.github.com (emphasis on the .com), please use <username>.github.io instead (emphasis on the .io). This doesn’t apply for GitHub-owned sites like pages.github.com, but does apply to user and project sites like twitter.github.com and parkr.github.com/my-project/.
Please give the whole post a read for schedules on brownouts where GitHub will turn off these redirects for a shorter period of time before turning it off completely on April 15, 2021.