Spider Web Woman Designs
10 Important Accelerated Mobile Pages (AMP) Components You Should Know - Accelerated Mobile Pages or AMP is Google’s initiative to make the mobile web faster.
Cross Device Web Site Development - Designing for the Complete Web (includes a list of testing tools)
Techniques for Responsive Typography - When it comes to responsive type on the web, there's more to do than just resizing the text's container and having the text reflow inside of it. This in-depth article covers various techniques for making text responsive.
Google's Mobile-Friendly Test
Microsoft Edge Site scan Run a quick code scan on any URL to check for out-of-date libraries, layout issues, and accessibility
Resize Window Plugin for Chrome - Resize browser window to emulate various screen resolutions
deviceponsive - show a quick preview of your website's responsive to different device resolutions like Macbook, iPad, or iPhone.
Test your websites in smartphone & tablet emulators - MobileTest.me
Mobile SEO testing - including a Mobile optimization guide 2015