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.