Currently the property "perspective" seems to expect a number in CSS-Sanitizer, but it should be a lenght.
See e.g. https://developer.mozilla.org/en-US/docs/Web/CSS/perspective

I guess the problem is around line 1016 in https://gerrit.wikimedia.org/r/plugins/gitiles/css-sanitizer/%2B/master/src/Sanitizer/StylePropertySanitizer.php
where it says:

$props['perspective'] = new Alternative( [ new KeywordMatcher( 'none' ), $n ] );

In the comment there it refers to a W3-standard, but maybe this is mistaken with the function "transform perspective()" which obviously is something different.

I came on this because in MediaWiki Extension TemplateStyles the following CSS statement gives an error when trying to save:

perspective: 1000px;