Cross Browser Testing Best Practices

I use a paid for in-browser software called crossbrowsertesting.com. It's worth its price many times over. You can test different 'screen sizes' by dragging the browser from left to right, assuming you're using media queries with min-width and max-width as opposed to min-device-width and max-device-width - that, however, only tests various breakpoints - you also have to test the various browsers out there such as IE, Firefox, Chrome and Safari, which are the 4 main browsers we need to cater for as web designers.

If you have Firefox, you should use a plugin called "Firebug" for designing/developing. You can use Firefox's built in responsive design view to test various screen sizes - go to Tools > Web Developer > Responsive Design View - here you can test various screen sizes to see how your media queries are appearing to the browsers you're currently in.

You should always do what you can to ensure your code is compatible with most versions of a browser; one browser i do not support no matter what is Internet Explorer 7 - it's the most ridiculous browser you will come across in regards to debugging and fixing responsiveness. Avoid supporting it like the plague, perhaps then people may decide to upgrade.

That said, only a small percent of people use IE7, but this changes depending on the website, the websites audience and the region they're located in. For example you will find a lot of Indian people still use IE6 and IE7 - so if you're developing for a client who has lots of users with IE7 - you should likely do your utmost to support it.