Render API overview and Render Element
The main purpose of Drupal's Theme system is to give themes complete control over the appearance of the site, which includes the markup returned from HTTP requests and the CSS files used to style that markup.
In order to ensure that a theme can completely customize the markup, module developers should avoid directly writing HTML markup for pages, blocks, and other user-visible output in their modules, and instead return structured "render arrays" (see Render arrays below).
