Building and rendering a table required writing a custom/dedicated theme function for most tables in Drupal 7.

Drupal 8 introduces a new element '#type' => 'table', which comes with built-in, optional support for TableDrag and TableSelect, and which requires no more custom plumbing for most use-cases.

Note: The following conversion example shows both TableDrag and TableSelect in a single table for brevity, but they are usually not combined in a single table.