HTML5-lib is added to Drupal 8 core for the filter system

Drupal's HTML filtering system now supports HTML5. The 'html' and 'htmlcorrector' filters handle the validation and cleaning responsibilities to any markup that was entered into text fields, whether managed by text editors or not. This key system provides a layer of security and validation that prevents many kinds of attacks while creating content the the administrative user interface.

In Drupal 7:

Drupal's html processing filters are driven by the libxml2 library provided by core PHP to clean and transform html to DOM. libxml2 has served us well but misinterprets some of HTML5 as invalid markup. Saved markup would, by default, adhere to the XHTML Strict specification (http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd)

In Drupal 8:

Drupal now includes the HTML5-php library from https://github.com/Masterminds/html5-php to advance the filtering system's understanding of HTML5 so that markup can be validated, cleaned, and converted into DOM properly. HTML5-php will take over the HTML to DOM processing duties from libxml2. As a result a lot of tests where updated to take advantage of the new capabilities.

No developer targeted APIs were impacted.