Originally Posted by Vytfla
If I am using some custom icon fonts from icomoon.io, do I have to specify somewhere on the site something like 'icon fonts from www.icomoon.io?
It all depends on what the agreement you have with them is regarding their use.
In this particular case the agreement can be found at http://creativecommons.org/licenses/by/3.0/
which suggests that you identify the agreement under which you are using the icons by linking to that specific page.
You really out to read the conditions for use before you use something that you don't have complete ownership of.