Nearly all browsers nowadays support CSS and many other applications do, too. To write CSS, you don't need more than a text editor, but there are many tools available that make it even easier.
Of course, nearly all software has bugs. And some programs are further ahead implementing the latest CSS modules than others. Various sites describe bugs and work-arounds.
Links to official feature lists of various products.
Several people maintain independent CSS support charts:
These sources document the level of support in various browsers:
Currently, most Web authoring tools provide some sort of support for CSS style sheets. The list below is far from complete, but contains (in chronological order) all tools that have been reported to us.
The AXR project aims to
replace HTML and CSS with XML and HSS (a language inspired by CSS):
no incompatibilities thanks to a single (Open Source)
implementation; support for simple transformations; delegation
& object orientation; no need for backwards compatibility with
HTML/CSS; advanced layout controls, shapes and splines, etc.
YesLogic has released Prince 7.1, a program to
produce PDF from HTML, MathML, SVG and generic XML. This version
adds PDF actions and some experimental features, such as Web Fonts in WOFF, a 'table-baseline' property
(especially useful for math), and a 'border-clip' property (to
suppress parts of a border). Prince offers many (standard or
proposed) CSS3 features, including hyphenation, rounded corners and
footnotes. (Windows, Mac OS X, Solaris, Linux (i386), BSD; free
personal license)
Version 3 of the Sass preprocessor
for CSS adds a new input syntax that is a pure superset of CSS.
This makes it easy to use Sass with existing style sheets. The
Python-style indented syntax is also still available. Can be used
stand-alone or integrated in Ruby-on-Rails. (Ruby, Open Source)