It is apparently also hard to maintain over time. I heavily customized my Firefox and Thunderbird and it is annoying how often GUI components which should be equal (e.g. toolbar buttons in different places) look the same* but have multiple different CSS rules.

Some widgets, or rather the repeated reinventions of widgets in HTML+CSS, use variables and the next equally looking widget has none, or other variables with the same values.

From an outsider perspective it looks like a mess.

* or almost the same with minimal, likely unintended, differences like one button has a slight border when hovered but another button right next to it has none