Dockerized portable standalone version of the validator.nu markup checker from https://github.com/validator/validator.github.io
Build with:
docker build -t myvalidator .
Run with:
docker run -d -p 8888:8888 myvalidator
Then visit http://localhost:8888
Should give an interface similar to what's seen on http://validator.nu/
Read about the validator: http://about.validator.nu/
The API is very useful: http://wiki.whatwg.org/wiki/Validator.nu_Web_Service_Interface