Updated README

README.md

-mincss is a CSS minifier that merges different groups of properties, for
-different selectors.
+mincss is an extendible CSS minifier written in OCaml. It contains a complete
+parser for the CSS3 language, along with consistent type definitions and a
+traversal utility function for use in transformation passes.
+
+mincss is currently still in development, finished components are the parser,
+stringification (along with whitespace compression), and color compression.
+Rulset merging is partially documented below but currently unimplemented.
+
+
+Features
+========
+
+- Whitespace compression
+- Color compression
+- Creation of shorthand properties (e.g. `font` and `background`)
+- Ruleset merging (see below)
+- Command-line interface and web interface
+
+
+Ruleset merging
+---------------
+
+Apart from simply writing the CSS in a shorter format (i.e.
+whitespace/color/shorthand compression), mincss attempts to restructure rule
+sets into a shorter variant. For example:
+
+    a { color: red }
+    p { color: red }
+
+can be written much shorter as:
+
+    a, p { color: red }
+
+Merging selectors is something that is done by the programmer in most cases,
+but the example above may occur when multiple different CSS files are merged
+into one, or when a large CSS file is structured in such a way that the
+definitions of `a` and `p` are far apart. A special case is when the same
+selector appears in different rulesets, which may happen when a framework
+stylesheet is merged with a page-specific stylesheet:
+
+    a {
+        color: blue;
+        font-weight: bold;
+    }
+
+    ...
+
+    a {
+        color: red;
+    }
+
+This is merged into:
+
+    a {
+        color: blue;
+        font-weight: bold;
+        color: red;
+    }
+
+Because the `color` property is now overwritten in the same ruleset, the early
+definition is removed:
+
+    a {
+        font-weight: bold;
+        color: red;
+    }