So you can enforce https on github pages by checking this checkbox here:
It takes some time to reflect.
Also, if it is not working, some of your css/JS files must be accessed over HTTP, so if any of the external dependencies of your website are requested over HTTP, change that to HTTPS and then it should work fine.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…