Hi Remco,
I am not sure exactly what you mean by 'System Texts,' but Mendix does have some widgets that can help. A few widgets that render HTML are:
HTML/ JavaScript Snippet, where you can hard code your HTML and have it render on the page
Bootstrap WYSIWYG Editor - This download contains widgets to both enter/edit html text as well as display it on a page. This is better if the text you want to display is an attribute of an entity.
Hope this helps,
Rob