[css-minifier] Don't remove the old browser fallback

[yisibl]

This issue come from: cssnano/cssnano#1163

.my-class {
  margin-right: 22px;
  margin-right: max(4%, 22px);
}

Output

.my-class {
  margin-right: max(4%, 22px);
}

Expected

.my-class {
  margin-right: 22px;
  margin-right: max(4%, 22px);
}

[devongovett]

Related to #109.

Not sure exactly how we'd know when it is safe to remove a duplicate declaration or not...

[yisibl]

Ideally, we could match whether a value is supported or not based on the caniuse data. But this certainly won't cover all cases, especially with the new features the browser has just implemented.

Maybe we need a parameter to let the user manually set which properties or values are not covered.

[nstepien]

I've found a similar issue in our codebase: playground

overflow: clip removes overflow: hidden, even if overflow: clip isn't supported by all targeted browsers.

IMO duplicate property removal should be disabled until this is fixed, as it's going to break existing styles.

[devongovett]

It's not so simple unfortunately. Removing duplicate properties isn't actually a feature. It is a side effect of merging longhands into shorthands. For example, combining overflow-x: hidden; overflow-y: hidden into just overflow: hidden. This involves looking at all of the declarations in a rule, since later ones override earlier ones. This is handled on a per-property basis.

To handle this properly, we'd need compatibility data on every CSS value. Here's how image-set was handled, for example: cb69f33.

[devongovett]

This is now supported for many more types of values such as lengths, colors, gradients, images, font styles, etc. Please let me know if I missed one.