Auto merge of #15972 - jryans:ps-contain, r=emilio

Parsing / serialization for CSS contain

Adds parsing / serialization for CSS contain to the style package.

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #15955
- [x] There are tests for these changes

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tests/unit/style/parsing/containment.rs

@@ -0,0 +1,26 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use cssparser::Parser;
+use media_queries::CSSErrorReporterTest;
+use style::parser::ParserContext;
+use style::stylesheets::Origin;
+
+#[test]
+fn contain_longhand_should_parse_correctly() {
+    use style::properties::longhands::contain;
+    use style::properties::longhands::contain::SpecifiedValue;
+
+    let none = parse_longhand!(contain, "none");
+    assert_eq!(none, SpecifiedValue::empty());
+
+    let strict = parse_longhand!(contain, "strict");
+    assert_eq!(strict, contain::STRICT);
+
+    let style_paint = parse_longhand!(contain, "style paint");
+    assert_eq!(style_paint, contain::STYLE | contain::PAINT);
+
+    // Assert that the `2px` is not consumed, which would trigger parsing failure in real use
+    assert_parser_exhausted!(contain, "layout 2px", false);
+}

tests/unit/style/parsing/mod.rs

@@ -87,6 +87,7 @@ mod basic_shape;
 mod border;
 mod box_;
 mod column;
+mod containment;
 mod effects;
 mod font;
 mod image;