Download and install Emscripten.
Execute build_js.py script:
python <opencv_src_dir>/platforms/js/build_js.py <build_dir>
If everything is fine, a few minutes later you will get <build_dir>/bin/opencv.js. You can add this into your web pages.
Find out more build options by -h switch.
For detailed build tutorial, check out <opencv_src_dir>/doc/js_tutorials/js_setup/js_setup/js_setup.markdown.