As a reminder, the rollup CSS and JS are compilations of many independently versioned components and is likely to contain changes that may disrupt the look of your site. We advise testing. Even better is to build your own CSS rollup from the npm sources, you can find a guide here.
Continuing work on React support
We've given further consideration on how to best support React. We'll use a dual approach, providing full true React components for complex VF components. For less interactive components we'll continue to consume the precompiled Nunjucks templates.
We're working to revitalize our CSS linter and as part of that we needed to update the stylelinter configuration to remove false positives. We also needed to fix a number of small CSS inconsistencies that had been introduced. A number of components were updated in pull request #1405.