Website evaluation tools

Tools to validate a website for accessibility.
AccessibilityNavigate the site with the keyboard only. Is it possible? Do you get a clear focus indication?
AccessibilityZoom in and magnify your pages by 200% or even 500%. Is the site still working and do you get (probably) a mobile friendly layout?
HTMLSwitch off CSSA good check for semantic HTML is to switch off CSS in the browser. The document should be usable and readable even without CSS attached.
AccessibilityFirefox Accessibility InspectorCheck the entire web page for visual contrast, navigation via keyboard, missing text labels, and visualize tabbing order. It also has a simulator that lets you see what a web page would look like to users with various forms of color vision deficiency, as well as contrast sensitivity loss.
AccessibilityIBM Equal Access Accessibility CheckerThere is even a node package of the checker available.
AccessibilityWebAIM WAVECheck accessibility patterns
AccessibilityWebAIM Contrast CheckerCheck contrast of two colors
AccessibilityColorslurpCheck contrast of two colors. Desktop tool for Mac.
AccessibilityColor Contrast Accessibility ValidatorCheck contrast of an entire web page
PerformanceLighthouse PageSpeed InsightsCheck web page performance. A Lighhouse analysis can also be started through the Chrome Developer Tools, where it is available as a dedicated top-level menu entry. This allows to analyse a website running locally on the developer machine.
HTMLW3C Markup Validation ServiceCheck HTML standard compliance