These are the websites I use to check my work when developing a new website or new page.
https://developers.google.com/speed/pagespeed/insights/
Googles page speed insights. I don’t like that it always complains about eliminating render blocking css,
even though my css file is tiny. It doesn’t seem to take file size into account at all.
http://tools.pingdom.com/fpt/
Pingdoms speed test - love this for checking how large the total download is and the score.
http://www.webpagetest.org/
Another speed test, like the filmstrip/video review.
https://validator.w3.org/nu/
Checks for valid code, sometimes I have an extra closing div or a missing alt tag, this checks for that. I probably should be doing this
in my build process but have not figured that out yet (htmlproofer?).
https://www.responsinator.com/
I like this cause you can see a bunch of screens all at once.
https://developers.facebook.com/tools/debug/
Checks your facebook og tags.
https://www.ssllabs.com/ssltest/index.html
If you are using SSL on your site this is a must - you should be able to get an A grade. Not getting an A may cause issues
in some browsers.