These are the websites I use to check my work when developing a new website or new page.
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.
Pingdoms speed test - love this for checking how large the total download is and the score.
Another speed test, like the filmstrip/video review.
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?).
I like this cause you can see a bunch of screens all at once.
Checks your facebook og tags.
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.