525icons by css-design.no is a very comprehensive and versatile web icon collection,
designed for websites and apps, and it is completely free for commercial use. (The license does not apply to copyrighted logos and brand icons.)
You may investigate the different css-codes in use, inside the file: demo.html and 525icons.css.
The icons in the set are optimized within a 16px grid. You will get crisp results by setting your font-size to 16px, 32px, 48px, etc.
The ux-2x - 7x classes are based on (16 × Npx) sizes.
In the OTF folder, you also will find the font in otf format, which you may install for use in PC or Mac applications.
Click on the icon below to view an example of the font in use inside of Illustrator: