哎 弄个域名绑定也弄了一天。
貌似绑定A级域名要用192.30.252.153才行了

照着一个外国大叔的配的：

1. Set the @ (used to denote the domain name for which you’re configuring the DNS) IP Address/URL to 192.30.252.153 and the Record Type to A (Address) with a TTL (an acronym for Time To Live that refers to the capability of the DNS servers to cache DNS records) of 1800.
2. Set the www (the subdomain www) IP Address/URL to username.github.io. (with trailing period) and the Record Type to CNAME (Alias) with a TTL of 1800.
3. In SUB-DOMAIN SETTINGS, add an @ in the first field, and duplicate the settings in Step 1, save for the IP Address/URL, which should be 192.30.252.154.

Save and then you’re all set! Please note, however that it may take some time for the changes to the DNS to propagate.

github官方文档：https://help.github.com/articles/setting-up-a-custom-domain-with-pages