Add code to your child theme’s functions.php file or via a plugin that allows custom functions to be added, such as the Code snippets plugin. Avoid adding custom code directly to your parent theme’s functions.php file as this will be wiped entirely when you update the theme.
By default, WordPress strips HTML from category descriptions. This code will prevent that from happening.
https://gist.github.com/woogists/7b275d76ba2c8750655a3b77f3cc6ac0
The first part prevents HTML from being stripped from term descriptions.
The second part prevents HTML being stripped out when using the term description function (http://codex.wordpress.org/Function_Reference/term_description).