Add support for transparent color value for *-color attributes (#214)

css/handlers.go

diff --git a/css/handlers.go b/css/handlers.go
index 41a00c8..6550d10 100644
--- a/css/handlers.go
+++ b/css/handlers.go
@@ -250,9 +250,9 @@
 		"writing-mode":               WritingModeHandler,
 		"z-index":                    ZIndexHandler,
 	}
-	colorValues = []string{"initial", "inherit", "aliceblue", "antiquewhite",
-		"aqua", "aquamarine", "azure", "beige", "bisque", "black",
-		"blanchedalmond", "blue", "blueviolet", "brown", "burlywood",
+	colorValues = []string{"transparent", "initial", "inherit", "aliceblue",
+		"antiquewhite", "aqua", "aquamarine", "azure", "beige", "bisque",
+		"black", "blanchedalmond", "blue", "blueviolet", "brown", "burlywood",
 		"cadetblue", "chartreuse", "chocolate", "coral", "cornflowerblue",
 		"cornsilk", "crimson", "cyan", "darkblue", "darkcyan", "darkgoldenrod",
 		"darkgray", "darkgrey", "darkgreen", "darkkhaki", "darkmagenta",

sanitize_test.go

diff --git a/sanitize_test.go b/sanitize_test.go
index 1e5607a..8e441bb 100644
--- a/sanitize_test.go
+++ b/sanitize_test.go
@@ -1748,6 +1748,10 @@
 			expected: `<div style="background-color: coral"></div>`,
 		},
 		{
+			in:       `<div style="background-color: transparent"></div>`,
+			expected: `<div style="background-color: transparent"></div>`,
+		},
+		{
 			in: `<div style="background-image: url('http://paper.gif')">` +
 				`</div><div style="background-image: inherit"></div>`,
 			expected: `<div style="background-image: ` +