There are many approaches https://en.wikipedia.org/wiki/Cluster_analysis. In the research k-means clustering was used. It's probably not the best clustering algo for this task and a much better results would be achieved if a different one was used or if some data manipulation was applied prior to the clustering. Here's k-means clustering explanation if you're interested https://youtu.be/_aWzGGNrcic
They are trying to predict what types of html pages cause parsing problems. They have a set of variables that describe an html page. They are trying to figure out which variable or combination of variables predict a bad parsing outcome.
Think of cluster analysis as simply plotting those variables as points on a graph. Then drawing a circle around points that are close to each other.
