The web is for everyone, but support is not the same as optimization.
Rather than trying to provide the same level of functionality for older browsers, Kraken uses progressive enhancement to serve a basic experience to all browsers (even Netscape and IE 5). Newer browsers that support modern APIs and techniques get a better layout, more visually attractive elements, and an enhanced experience.
Kraken works in all browsers, but it's optimized for modern browsers and IE 9+.
Kraken uses Autoprefixer, and is configured to only add prefixes if required by the last two versions of a browser.
If a feature isn't working (for example, the grid does not work in Firefox 28 and lower), it may simply need a vendor prefix. You can add these manually, or adjust the Autoprefixer settings in
gulpfile.js if you're working with the source code.
For more details on when support for specific features were added to different browsers, consult Can I Use.