css: add README

src/css/README.md

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+# css
+
+Lightpanda css implements CSS selectors parsing and matching in Zig.
+This package is a port of the Go lib [andybalholm/cascadia](https://github.com/andybalholm/cascadia).
+
+## Usage
+
+### Query parser
+
+```zig
+const css = @import("css.zig");
+
+const selector = try css.parse(alloc, "h1", .{});
+defer selector.deinit(alloc);
+```
+
+### DOM tree match
+
+The lib expects a `Node` interface implementation to match your DOM tree.
+
+```zig
+pub const Node = struct {
+    pub fn firstChild(_: Node) !?Node {
+        return error.TODO;
+    }
+
+    pub fn lastChild(_: Node) !?Node {
+        return error.TODO;
+    }
+
+    pub fn nextSibling(_: Node) !?Node {
+        return error.TODO;
+    }
+
+    pub fn prevSibling(_: Node) !?Node {
+        return error.TODO;
+    }
+
+    pub fn parent(_: Node) !?Node {
+        return error.TODO;
+    }
+
+    pub fn isElement(_: Node) bool {
+        return false;
+    }
+
+    pub fn isDocument(_: Node) bool {
+        return false;
+    }
+
+    pub fn isComment(_: Node) bool {
+        return false;
+    }
+
+    pub fn isText(_: Node) bool {
+        return false;
+    }
+
+    pub fn isEmptyText(_: Node) !bool {
+        return error.TODO;
+    }
+
+    pub fn tag(_: Node) ![]const u8 {
+        return error.TODO;
+    }
+
+    pub fn attr(_: Node, _: []const u8) !?[]const u8 {
+        return error.TODO;
+    }
+
+    pub fn eql(_: Node, _: Node) bool {
+        return false;
+    }
+};
+```
+
+You also need do define a `Matcher` implementing a `match` function to
+accumulate the results.
+
+```zig
+const Matcher = struct {
+    const Nodes = std.ArrayList(Node);
+
+    nodes: Nodes,
+
+    fn init(alloc: std.mem.Allocator) Matcher {
+        return .{ .nodes = Nodes.init(alloc) };
+    }
+
+    fn deinit(m: *Matcher) void {
+        m.nodes.deinit();
+    }
+
+    pub fn match(m: *Matcher, n: Node) !void {
+        try m.nodes.append(n);
+    }
+};
+```
+
+Then you can use the lib itself.
+
+```zig
+var matcher = Matcher.init(alloc);
+defer matcher.deinit();
+
+try css.matchAll(selector, node, &matcher);
+_ = try css.matchFirst(selector, node, &matcher); // returns true if a node matched.
+```
+
+## Features
+
+* [x] parse query selector
+* [x] `matchAll`
+* [x] `matchFirst`
+* [ ] specificity
+
+### Selectors implemented
+
+#### Selectors
+
+* [x] Class selectors
+* [x] Id selectors
+* [x] Type selectors
+* [x] Universal selectors
+* [ ] Nesting selectors
+
+#### Combinators
+
+* [x] Child combinator
+* [ ] Column combinator
+* [x] Descendant combinator
+* [ ] Namespace combinator
+* [x] Next-sibling combinator
+* [x] Selector list combinator
+* [x] Subsequent-sibling combinator
+
+#### Attribute
+
+* [x] `[attr]`
+* [x] `[attr=value]`
+* [x] `[attr|=value]`
+* [x] `[attr^=value]`
+* [x] `[attr$=value]`
+* [ ] `[attr*=value]`
+* [x] `[attr operator value i]`
+* [ ] `[attr operator value s]`
+
+#### Pseudo classes
+
+* [ ] `:active`
+* [ ] `:any-link`
+* [ ] `:autofill`
+* [ ] `:blank Experimental`
+* [x] `:checked`
+* [ ] `:current Experimental`
+* [ ] `:default`
+* [ ] `:defined`
+* [ ] `:dir() Experimental`
+* [x] `:disabled`
+* [x] `:empty`
+* [x] `:enabled`
+* [ ] `:first`
+* [x] `:first-child`
+* [x] `:first-of-type`
+* [ ] `:focus`
+* [ ] `:focus-visible`
+* [ ] `:focus-within`
+* [ ] `:fullscreen`
+* [ ] `:future Experimental`
+* [x] `:has() Experimental`
+* [ ] `:host`
+* [ ] `:host()`
+* [ ] `:host-context() Experimental`
+* [ ] `:hover`
+* [ ] `:indeterminate`
+* [ ] `:in-range`
+* [ ] `:invalid`
+* [ ] `:is()`
+* [x] `:lang()`
+* [x] `:last-child`
+* [x] `:last-of-type`
+* [ ] `:left`
+* [x] `:link`
+* [ ] `:local-link Experimental`
+* [ ] `:modal`
+* [x] `:not()`
+* [x] `:nth-child()`
+* [x] `:nth-last-child()`
+* [x] `:nth-last-of-type()`
+* [x] `:nth-of-type()`
+* [x] `:only-child`
+* [x] `:only-of-type`
+* [ ] `:optional`
+* [ ] `:out-of-range`
+* [ ] `:past Experimental`
+* [ ] `:paused`
+* [ ] `:picture-in-picture`
+* [ ] `:placeholder-shown`
+* [ ] `:playing`
+* [ ] `:read-only`
+* [ ] `:read-write`
+* [ ] `:required`
+* [ ] `:right`
+* [x] `:root`
+* [ ] `:scope`
+* [ ] `:state() Experimental`
+* [ ] `:target`
+* [ ] `:target-within Experimental`
+* [ ] `:user-invalid Experimental`
+* [ ] `:valid`
+* [ ] `:visited`
+* [ ] `:where()`
+* [ ] `:contains()`
+* [ ] `:containsown()`
+* [ ] `:matched()`
+* [ ] `:matchesown()`
+* [x] `:root`
+