If you run a website, even if you know nothing else about optimising for mobile use, please add the following line to your <head> section of your pages.
<meta name="viewport" content="width=device-width, initial-scale=1">
To get an idea of the difference this makes, visit this page, and a copy of this page without the line added from a mobile device.
You can learn more about this, as well as other tips for mobile optimisation from Google's web fundamentals pages
Notice anything wrong? Want to make an improvement? Open a pull request
<a href="//meta-name-viewport-content-width-device-width-initial-scale-1.com/"><img src="//meta-name-viewport-content-width-device-width-initial-scale-1.com/images/80x15.png"></a>