Go to github.io of a github repo which has gh-pages branch.

gotoGithubIO-bookmarklet.html

<a class="btn btn-default btn-info bookmarklet" href="javascript:(function(d,b,c,a){b=d.location;a=b.pathname.split('/');c=b.hostname;return'github.com'==c&&2<a.length?(a=b.protocol+'//'+a[1]+'.'+c.replace(/\.com$/,'.io')+'/'+a[2],void d.open(a,'_blank')):alert('Go to a github repo and try again!'),!1})(this);">
    Go to Github.IO
</a>

gotoGithubIO.js

/**
 * Go to github.io of a github repo which has gh-pages branch.
 *
 * @license MIT
 * @version 1.0.0
 * @author Dumitru Uzun (DUzun.Me)
 * @homepage https://duzun.me/playground#gotoGithubIO
 */
(function (w, l, h, p) {
    l = w.location;
    p = l.pathname.split('/');
    h = l.hostname;
    return h == 'github.com' && p.length > 2
        ? ( p=l.protocol + '//' + p[1] + '.' + h.replace(/\.com$/, '.io') + '/' + p[2],
        void w.open(p, '_blank')
        // l.href=p
        ) : alert('Go to a github repo and try again!'), false
    ;
}(this));

gotoGithubIO.min.js

javascript:(function(d,b,c,a){b=d.location;a=b.pathname.split('/');c=b.hostname;return'github.com'==c&&2<a.length?(a=b.protocol+'//'+a[1]+'.'+c.replace(/\.com$/,'.io')+'/'+a[2],void d.open(a,'_blank')):alert('Go to a github repo and try again!'),!1})(this);