Kdyby/HtmlValidatorPanel requires PHP 5.4 or higher
The best way to install Kdyby/HtmlValidatorPanel is using Composer:
$ composer require kdyby/html-validator-panel- enable in config
extensions:
htmlPanel: Kdyby\Extension\Diagnostics\HtmlValidator\DI\ValidatorExtension- profit
Homepage http://www.kdyby.org and repository http://github.com/Kdyby/HtmlValidatorPanel.