Accuracy testing Before the patch Copy the 2 attached files test.php and test.css to your moodle root direcoty. From command line, run: php test.php Look to the generated test.min.css file Verify that it has not been minified (it has multiple lines, like the original had, but with some details). Detail 1: Verify that the comment /* First comment */ is still there. Detail 2: Verify that the very last line that has three occurrences of .lots .of .css .rules all-together. Detail 3: Verify that there is only one /* @License MUT 2020 */ line. Detail 4: Verify that the /* Another comment */ is gone. After the patch From command line, run: php test.php again Look to the generated test.min.css file Verify that now the file has been minified more (styles don't have multiple lines anymore). Verify that the lines having styles have 1, 2 and 3 occurrences of the .lots .of .css .rules in each line. Verify that the /* First comment / and / Another comment */ are gone. Verify that the /* @preserve I've been preserved! */ is present in a line. Verify that there are two /* @License MUT 2020 */ lines. Exploratory testing Access to moodle, purge caches, switch between themes and visit some pages (course, wiki, forum...) verify that there isn't anything visually broken.