New versions of Sass are as backwards-compatible as possible, but sometimes a breaking change is necessary. Sass needs to keep up with the evolving CSS specification, and old language design mistakes occasionally need to be fixed.
Before each breaking change is released, Sass implementations will produce deprecation warnings for stylesheets whose behavior will change. Whenever possible, these warnings will include suggestions for how to update the deprecated styles to make them forward-compatible.
Different implementations have different policies for breaking changes and deprecations. Dart Sass will emit deprecation warnings for at least three months before releasing a breaking change, and will release the breaking change with a new major version number unless that change is necessary for CSS compatibility. CSS compatibility changes are often both non-disruptive and time-sensitive, so they may be released with new minor version numbers instead.
These breaking changes are coming soon or have recently been released:
Color functions are becoming more strict about units beginning in Dart Sass 1.32.0.
Compound selectors could not be extended in Dart Sass 1.0.0 and Ruby Sass 4.0.0.
The syntax for CSS custom property values changed in Dart Sass 1.0.0, LibSass 3.5.0, and Ruby Sass 3.5.0.