My graphics design skills are not going to win any prizes. To help me not make a complete fool of myself at website design I use these three tools to check for obvious failures:

1. The audit function built into Chrome's developer tools
2. The W3C unified validator at: https://validator.w3.org/unicorn/
3. Google's page speed tester (it actually checks layout and mobile errors too) at: https://developers.google.com/speed/pagespeed/insights/