Older browsers used to be very inconsistent in how they handled CSS. They not only had vendor-specific prefixes such as -WebKit-transition but also had their own quirks. Newer browsers follow modern standards better.

Now, we also have more powerful layout models such as flexbox, which reduce the complexity of code. All these have resulted in some very lightweight CSS frameworks.

For instance, Pure.css is only 3.8 KB minified and gzipped, but packed with features. Similarly, mini.css designed with mobile devices and modern browsers in mind is under 7 KB gzipped. For comparison, Bootstrap is 25 KB, gzipped, with all modules included.

While these lightweight frameworks might save some initial page load time, be sure to test them with all the different browsers your target users might use. Tools such as CanIUse.com can help by showing which features are supported across browsers and platforms. Bootstrap is quite good at maintaining backward compatibility with the widest range of clients.