HTML Purifier is a standards-compliant HTML filter library written in PHP. This library clean HTML code and converts it to standards compliant.
But if you need to use some custom content or code in your product, category, cms pages description or etc. HTML purifier can affects on your code and it will not works.
PrestaShop allows to disable HTML Purifier.
Open page: Shop Parameters -> General and disable feature: Use HTMLPurifier Library