LibWeb: Maintain easing keywords as KeywordStyleValue until use-time

This excludes `step-end` and `step-start` which are expected to be
converted to the equivalent function at parse time.

We are expected to serialize these as the explicit keywords - previously
we would parse as `EasingStyleValue` and serialize equivalent functions
as the keywords. This caused issues as we would incorrectly serialize
even explicit functions as the keyword.

This also allows us to move the magic easing functions to
`EasingFunction` rather than `EasingStyleValue` which is a bit tidier

Libraries/LibWeb/CSS/EasingFunction.cpp

@@ -8,6 +8,7 @@
 #include <AK/BinarySearch.h>
 #include <LibWeb/CSS/StyleValues/EasingStyleValue.h>
 #include <LibWeb/CSS/StyleValues/IntegerStyleValue.h>
+#include <LibWeb/CSS/StyleValues/KeywordStyleValue.h>
 #include <LibWeb/CSS/StyleValues/NumberStyleValue.h>
 #include <LibWeb/CSS/StyleValues/PercentageStyleValue.h>
 
@@ -268,6 +269,41 @@ static Vector<LinearEasingFunction::ControlPoint> canonicalize_linear_easing_fun
     return canonicalized_control_points;
 }
 
+// https://drafts.csswg.org/css-easing-2/#linear-easing-function
+EasingFunction EasingFunction::linear()
+{
+    // Equivalent to linear(0, 1)
+    return LinearEasingFunction { { { 0, 0 }, { 1, 1 } }, "linear"_string };
+}
+
+// https://drafts.csswg.org/css-easing-2/#valdef-cubic-bezier-easing-function-ease-in
+EasingFunction EasingFunction::ease_in()
+{
+    // Equivalent to cubic-bezier(0.42, 0, 1, 1).
+    return CubicBezierEasingFunction { 0.42, 0, 1, 1, "ease-in"_string };
+}
+
+// https://drafts.csswg.org/css-easing-2/#valdef-cubic-bezier-easing-function-ease-out
+EasingFunction EasingFunction::ease_out()
+{
+    // Equivalent to cubic-bezier(0, 0, 0.58, 1).
+    return CubicBezierEasingFunction { 0, 0, 0.58, 1, "ease-out"_string };
+}
+
+// https://drafts.csswg.org/css-easing-2/#valdef-cubic-bezier-easing-function-ease-in-out
+EasingFunction EasingFunction::ease_in_out()
+{
+    // Equivalent to cubic-bezier(0.42, 0, 0.58, 1).
+    return CubicBezierEasingFunction { 0.42, 0, 0.58, 1, "ease-in-out"_string };
+}
+
+// https://drafts.csswg.org/css-easing-2/#valdef-cubic-bezier-easing-function-ease
+EasingFunction EasingFunction::ease()
+{
+    // Equivalent to cubic-bezier(0.25, 0.1, 0.25, 1).
+    return CubicBezierEasingFunction { 0.25, 0.1, 0.25, 1, "ease"_string };
+}
+
 EasingFunction EasingFunction::from_style_value(StyleValue const& style_value)
 {
     auto const resolve_number = [](StyleValue const& style_value) {
@@ -335,6 +371,22 @@ EasingFunction EasingFunction::from_style_value(StyleValue const& style_value)
             });
     }
 
+    switch (style_value.to_keyword()) {
+    case Keyword::Linear:
+        return EasingFunction::linear();
+    case Keyword::EaseIn:
+        return EasingFunction::ease_in();
+    case Keyword::EaseOut:
+        return EasingFunction::ease_out();
+    case Keyword::EaseInOut:
+        return EasingFunction::ease_in_out();
+    case Keyword::Ease:
+        return EasingFunction::ease();
+    default: {
+        VERIFY_NOT_REACHED();
+    }
+    }
+
     VERIFY_NOT_REACHED();
 }