exp/template/html: simplify URL filtering

This removes a few cases from escapeAction and clarifies the
responsibilities of urlFilter which no longer does any
escaping or normalization.  It is now solely a filter.

R=nigeltao
CC=golang-dev
https://golang.org/cl/5162043

src/pkg/exp/template/html/escape.go

@@ -171,7 +171,7 @@ func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
 			switch c.state {
 			case stateCSSDqStr, stateCSSSqStr:
 				s = append(s, "exp_template_html_cssescaper")
-			case stateCSSDqURL, stateCSSSqURL, stateCSSURL:
+			default:
 				s = append(s, "exp_template_html_urlnormalizer")
 			}
 		case urlPartQueryOrFrag:

src/pkg/exp/template/html/escape_test.go

@@ -155,7 +155,7 @@ func TestEscape(t *testing.T) {
 		{
 			"nonHierURL",
 			`<a href={{"mailto:Muhammed \"The Greatest\" Ali <m.ali@example.com>"}}>`,
-			`<a href=mailto:Muhammed&#32;&#34;The&#32;Greatest&#34;&#32;Ali&#32;&lt;m.ali@example.com&gt;>`,
+			`<a href=mailto:Muhammed%20%22The%20Greatest%22%20Ali%20%3cm.ali@example.com%3e>`,
 		},
 		{
 			"urlPath",
@@ -352,9 +352,15 @@ func TestEscape(t *testing.T) {
 		},
 		{
 			"styleStrBadProtocolBlocked",
-			`<a style="background: '{{"javascript:alert(1337)"}}'">`,
+			`<a style="background: '{{"vbscript:alert(1337)"}}'">`,
 			`<a style="background: '#ZgotmplZ'">`,
 		},
+		{
+			"styleStrEncodedProtocolEncoded",
+			`<a style="background: '{{"javascript\\3a alert(1337)"}}'">`,
+			// The CSS string 'javascript\\3a alert(1337)' does not contains a colon.
+			`<a style="background: 'javascript\\3a alert\28 1337\29 '">`,
+		},
 		{
 			"styleURLGoodProtocolPassed",
 			`<a style="background: url('{{"http://oreilly.com/O'Reilly Animals(1)<2>;{}.html"}}')">`,

src/pkg/exp/template/html/url.go

@@ -10,15 +10,14 @@ import (
 	"strings"
 )
 
-// urlFilter returns the HTML equivalent of its input unless it contains an
-// unsafe protocol in which case it defangs the entire URL.
+// urlFilter returns its input unless it contains an unsafe protocol in which
+// case it defangs the entire URL.
 func urlFilter(args ...interface{}) string {
 	s, t := stringify(args...)
 	if t == contentTypeURL {
-		return urlProcessor(true, s)
+		return s
 	}
-	i := strings.IndexRune(s, ':')
-	if i >= 0 && strings.IndexRune(s[:i], '/') < 0 {
+	if i := strings.IndexRune(s, ':'); i >= 0 && strings.IndexRune(s[:i], '/') < 0 {
 		protocol := strings.ToLower(s[:i])
 		if protocol != "http" && protocol != "https" && protocol != "mailto" {
 			return "#" + filterFailsafe