github.io is now the domain for GitHub's web customers' sites.
If you are on a *.github.COM page, you will be on a GitHub official page;
If you are on a *.github.IO page, you will be on a third party customers' page.
While github.com becomes more safe, any *.github.io pages will be under the control of the third party.
Google Safe Browsing currently says:
"What is the current listing status for github.io?
Part of this site was listed for suspicious activity 20 time(s) over the past 90 days.
What happened when Google visited this site?
Of the 12716 pages we tested on the site over the past 90 days, 71 page(s) resulted in malicious software being downloaded and installed without user consent. The last time Google visited this site was on 2015-08-05, and the last time suspicious content was found on this site was on 2015-08-05.
Malicious software includes 1138 virus, 6 scripting exploit(s), 4 trojan(s).
Malicious software is hosted on 4 domain(s), including qinyuanpei.com/, s3.amazonaws.com/github-cloud/, zief.pl/.
4 domain(s) appear to be functioning as intermediaries for distributing malware to visitors of this site, including yu0123456.com/, clicksor.net/, multihits.biz/.
This site was hosted on 3 network(s) including AS54113 (FASTLY), AS15169 (GOOGLE), AS14618 (AMAZON-AES).
Has this site acted as an intermediary resulting in further distribution of malware?
Over the past 90 days, github.io appeared to function as an intermediary for the infection of 4 site(s) including vkplugin.ru/, vksaverplus.ru/, qinyuanpei.github.com/.
Has this site hosted malware?
Yes, this site has hosted malicious software over the past 90 days. It infected 5 domain(s), including vkplugin.ru/, vksaverplus.ru/, trollbar.net/."
As you can see, while only 71 of the 12k+ pages hosted via github.io caused issues, these 71 pages included 1138 viruses, 6 exploits and 4 trojans.
Therefore, whilst most of github's customers keep clean sites, unfortunately, a few really don't - because of those few, I cannot give github.io a green light.
Use by all means, but be very wary!
On a side note - GitHub do seem to be constantly working to keep these issues a bay, but they can't get to them all quickly enough, so do take care.