Font Awesome
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fa-check-square-o · Unicode: f046 · Created: v1.0 · Categories: Web Application Icons, Form Control Icons

After you get up and running, you can place Font Awesome icons just about anywhere with the <i> tag:

Example of check-square-o fa-check-square-o 
<i class="fa fa-check-square-o" aria-hidden="true"></i>

Note: to improve web accessibility, we recommend using aria-hidden="true" to hide icons used purely for decoration.

Looking for more? Check out the examples.