readme update

Author: leeoniya

README.md

@@ -5,7 +5,7 @@ An exceptionally fast, thorough and tiny unused-CSS cleaner _(MIT Licensed)_
 ---
 ### Introduction
 
-DropCSS is an [exceptionally fast](#performance), thorough and tiny ([~9 KB min](https://github.com/leeoniya/dropcss/tree/master/dist/dropcss.min.js)) unused-CSS cleaner; it takes your HTML and CSS as input and returns only the used CSS as output. Its custom HTML and CSS parsers are highly optimized for the 99% use case and thus avoid the overhead of handling malformed markup or stylesheets, so you must provide well-formed input. There is minimal handling for complex escaping rules, so there will always exist cases of valid input that cannot be processed by DropCSS; for these infrequent cases, please [start a discussion](https://github.com/leeoniya/dropcss/issues).
+DropCSS is an [exceptionally fast](#performance), thorough and tiny ([~10 KB min](https://github.com/leeoniya/dropcss/tree/master/dist/dropcss.min.js)) unused-CSS cleaner; it takes your HTML and CSS as input and returns only the used CSS as output. Its custom HTML and CSS parsers are highly optimized for the 99% use case and thus avoid the overhead of handling malformed markup or stylesheets, so you must provide well-formed input. There is minimal handling for complex escaping rules, so there will always exist cases of valid input that cannot be processed by DropCSS; for these infrequent cases, please [start a discussion](https://github.com/leeoniya/dropcss/issues).
 
 As a bonus, DropCSS will also remove unused `@keyframes` and `@font-face` blocks - an out-of-scope, purely intra-CSS optimization. Speaking of which, it's a good idea to run your CSS through a structural optimizer like [clean-css](https://github.com/jakubpawlowicz/clean-css), [csso](https://github.com/css/csso), [cssnano](https://github.com/cssnano/cssnano) or [crass](https://github.com/mattbasta/crass) to re-group selectors, merge redundant rules, etc. It probably makes sense to do this after DropCSS, which can leave redundant blocks, e.g. `.foo, .bar { color: red; } .bar { width: 50%; }` -> `.bar { color: red; } .bar { width: 50%; }` if `.foo` is absent from your markup.
 
@@ -74,6 +74,27 @@ The `shouldDrop` hook is called for every CSS selector that could not be matched
 ### Features
 
 - Retention of all transient pseudo-class and pseudo-element selectors which cannot be deterministically checked from the parsed HTML.
+- Deep resolution of composed [CSS variables](https://developer.mozilla.org/en-US/docs/Web/CSS/--*), e.g:
+  ```css
+  :root {
+    --font-style: italic;
+    --font-weight: bold;
+    --line-height: var(--height)em;
+    --font-family: 'Open Sans';
+    --font: var(--font-style) var(--font-weight) 1em/var(--line-height) var(--font-family);
+    --height: 1.6;
+  }
+
+  @font-face {
+    font-family: var(--font-family);
+    src: url("/fonts/OpenSans-Regular-webfont.woff2") format("woff2"),
+         url("/fonts/OpenSans-Regular-webfont.woff") format("woff");
+  }
+
+  body {
+    font: var(--font);
+  }
+  ```
 - Supported selectors
   - `*` - universal
   - `<tag>` - tag