This could be becasue you have set a minimum font size for your browser.
Check your browser config, it has a section about minimum font size, probably it’s something like 8px, so this 5px will be ignored, and test fails.
Example in chrome:
Go to menu -> settings -> search for and goto “Customize fonts” -> adjust the minimum font size to something like 4px and try again the test. Later you would rollback this setting of course.