The WebKit Tests
Before patches can land in any of the frameworks in the repository, the layout regression tests must be run. To run these tests, do the following:
Download the Ahem font.
This special simple font is used to construct certain types of predictable tests. A page on Ian Hickson's site has a more detailed explanation.
Open the downloaded file in the Font Book application and click on the Install Font button to install it.
Execute the run-webkit-tests script.
It's handy to put the WebKitTools/Scripts directory in your shell path so you can type commands like build-webkit without specifying the path to the script.
The script will dump the render trees for all of the pages and diff the results against the expected correct results. If no differences are found, then the patch has passed the tests. If any tests fail, then the patch cannot be committed until the discrepancies in the tests are resolved.
Regression tests must be run on Tiger, since the expected results checked in were generated on Tiger.
How to run the tests
The script will run all the tests and summarize how the results differ from what is currently expected.
What just happened
After all the test runs have finished the results of tests are saved to actual.html. The script the compares these results from your local tree against what is expected to pass/fail from the tip of tree. If there are any regressions caused by your changes you'll be made aware of them. If you fixed a bug that caused an existing failure, you'll also be made aware of what specific test your fix affected.
What to do next
So you fixed a bug that fixed a test. This means you've now set a new baseline for the tree as a result.
cp actual.html expected.html
When you land your changes, the baseline (expected.html) will be updated in the tree.
What happens if I caused a regression?
It's not the end of the world. Go back and fix your bug and re-run run-mozilla-tests as many times as necessary.