We now do most of our development using Google Chrome. We also test extensively using Firefox, and to a lesser extent, IE, Edge, and Safari. If you find a problem with any of our extensions using any of those browsers, please tell us. We don't routinely test in Opera but we will attempt to fix reported issues.
At the front end, all of our extensions should produce HTML that validates as HTML5. If you discover an HTML validation error, please tell us and we will fix it. We don't worry so much about validating the back end, but if you find something that renders incorrectly, please tell us and we will fix it. We validate to CSS 3.0 where possible.
We develop and test using PHP 5.3, 5.5 and 7 on Mac, Windows and Linux. We have PHP error reporting set to E_ALL | E_STRICT, which means that our released code should not issue any errors or notices. If you do see any errors or notices, please tell us and we will fix them.
Joomla 3 now supports other databases besides just MySql. Our extensions only support MySql. If this is an issue for you, please let us know. If we receive a significant number of requests to support different databases, we will have to decide what to do. We develop and test using MySql 5.5 and 5.6.