According to DMR, updated August 14th, 2015
WordPress is licensed under the GPLv2 or later which guarantees users several freedoms:
From WordPress website
WordPress is guided by the following set of principles:
From WordPress website
WordPress developers allow the following requirements:
It is advised to use the latest supported versions of both PHP and MySQL to avoid attacks with exposed security vulnerabilities
But still... use them for personal blogs and website, contribute back when finding issues on them.
May solve some of the problem, but definitely not a fully professional solution.
Creating a new theme consists in creating all the required elements to display the WorPress website. Therefore, it implies:
Creating a child theme consists in extending / overwriting an existing theme. Therefore it implies:
The WordPress offers lessons on the codex (ie: their documentation)