Add a possibility to run SCSS, Sass and other CSS preprocessors (LESS, Stylus, PostCSS, maybe others) in code snippets. Many developers use CSS preprocessors. This will remove need to convert demos to plain CSS and/or adding links to third-party sites which support this (like CodePen) just to demonstrate code examples.
CSS Preprocessors and MCVE
Some people downvoted this suggestion because they think that this will encourage people to post non-minimal examples. I don't agree with this.
Often people post pretty minimal Sass/SCSS because they are just using it. I know that I can edit question and translate Sass/SCSS to CSS or ask OP to do it. But I think this is really redundant action. If question is not MCVE, it should be closed/deleted/downvoted but lack of this possibility will encourage people to post links to sites like CodePen because our code snippet don't have many features.