As a website developer, our aim should be to make our content accessible to a maximum number of people and for that, our code should be bug-free and syntactically and semantically correct. Here are some of the best tools I've come across:
The W3C validator is great. You can validate your code either by directly uploading the file or the URL(Uniform Resource Locator) or by direct input. Always use a validator after finishing writing the code as this will help you to write efficient and error-free code.
WAVE Web Accessibility Evaluation Tool
Wave is an amazing tool as it helps you make your website accessible to a maximum number of people. It also helps you find errors that might normally stay hidden.
Funkify is an extension for Chrome that helps you experience the web and interfaces through the eyes of extreme users with different abilities and disabilities. Add this extension to your chrome and write better accessible code.
If you have other suggestions, kindly put them in the comments.