Here are some steps for using the process of elimination to create a minimal testcase from a webpage that displays a consistent bug.
# Save the webpage on your local hard drive. Make a second copy.
# From the first copy, cut out big pieces of CSS, JS or HTML -- start with the pieces that are most likely to not have the bug. For example, at first just try to strip out plain paragraphs as these are relatively simple.
# If the new version of the file still has the bug, save it as your second copy and repeat step 2.
# If the new version does not have the bug, then your second copy still contains the bug. In this case, repeat step 2 but avoid cutting out the same piece that you did last time.
For example, "No borders in tables at http://www.sillystatsandstuff.com"
can become something much more precise such as "tables with rowspans do not get borders".
Keep in mind that you need to use your brain throughout this process. For example,if the bug is layout flow related, then the width of your window and font size mayimpact the results.
== How to Help with Regressions -- Finding Regression Windows ==