We can use httpie to make requests to the W3C validator.
For example, to test the file index.html, run
https "https://validator.w3.org/nu/?out=gnu" @index.html "Content-Type: text/html; charset=utf-8" --print 'b'and any errors or warnings will be printed to STDOUT.
You can also get the error format in JSON by using the out=json syntax.
Another option which can be done locally is to use the python html5validator.
For example, if all your files are in a directory called html, calling
html5validator --root html/ --also-check-css --show-warningswill check all .html files as well ass .css files in that directory, and print warnings as well as errors.