exp/template/html: allow quotes on either side of conditionals and dynamic HTML names

This addresses several use cases:

(1) <h{{.HeaderLevel}}> used to build hierarchical documents.
(2) <input on{{.EventType}}=...> used in widgets.
(3) <div {{" dir=ltr"}}> used to embed bidi-hints.

It also makes sure that we treat the two templates below the same:

<img src={{if .Avatar}}"{{.Avatar}}"{{else}}"anonymous.png"{{end}}>
<img src="{{if .Avatar}}{{.Avatar}}{{else}}anonymous.png{{end}}">

This splits up tTag into a number of sub-states and adds testcases.

R=nigeltao
CC=golang-dev
https://golang.org/cl/5043042

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

@@ -19,11 +19,15 @@ type (
 	CSS string
 
 	// HTML encapsulates a known safe HTML document fragment.
-	// Should not be used for HTML from a third-party, or HTML with
+	// It should not be used for HTML from a third-party, or HTML with
 	// unclosed tags or comments. The outputs of a sound HTML sanitizer
 	// and a template escaped by this package are fine for use with HTML.
 	HTML string
 
+	// HTMLAttr encapsulates an HTML attribute from a trusted source,
+	// for example: ` dir="ltr"`.
+	HTMLAttr string
+
 	// JS encapsulates a known safe EcmaScript5 Expression, or example,
 	// `(x + y * z())`. 
 	// Template authors are responsible for ensuring that typed expressions
@@ -56,6 +60,7 @@ const (
 	contentTypePlain contentType = iota
 	contentTypeCSS
 	contentTypeHTML
+	contentTypeHTMLAttr
 	contentTypeJS
 	contentTypeJSStr
 	contentTypeURL
@@ -71,6 +76,8 @@ func stringify(args ...interface{}) (string, contentType) {
 			return string(s), contentTypeCSS
 		case HTML:
 			return string(s), contentTypeHTML
+		case HTMLAttr:
+			return string(s), contentTypeHTMLAttr
 		case JS:
 			return string(s), contentTypeJS
 		case JSStr:

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

@@ -16,6 +16,7 @@ func TestTypedContent(t *testing.T) {
 		`<b> "foo%" O'Reilly &bar;`,
 		CSS(`a[href =~ "//example.com"]#foo`),
 		HTML(`Hello, <b>World</b> &amp;tc!`),
+		HTMLAttr(` dir="ltr"`),
 		JS(`c && alert("Hello, World!");`),
 		JSStr(`Hello, World & O'Reilly\x21`),
 		URL(`greeting=H%69&addressee=(World)`),
@@ -38,6 +39,7 @@ func TestTypedContent(t *testing.T) {
 				`ZgotmplZ`,
 				`ZgotmplZ`,
 				`ZgotmplZ`,
+				`ZgotmplZ`,
 			},
 		},
 		{
@@ -50,6 +52,7 @@ func TestTypedContent(t *testing.T) {
 				`ZgotmplZ`,
 				`ZgotmplZ`,
 				`ZgotmplZ`,
+				`ZgotmplZ`,
 			},
 		},
 		{
@@ -59,11 +62,25 @@ func TestTypedContent(t *testing.T) {
 				`a[href =~ &#34;//example.com&#34;]#foo`,
 				// Not escaped.
 				`Hello, <b>World</b> &amp;tc!`,
+				` dir=&#34;ltr&#34;`,
 				`c &amp;&amp; alert(&#34;Hello, World!&#34;);`,
 				`Hello, World &amp; O&#39;Reilly\x21`,
 				`greeting=H%69&amp;addressee=(World)`,
 			},
 		},
+		{
+			`<a{{.}}>`,
+			[]string{
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				// Allowed and HTML escaped.
+				` dir="ltr"`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+				`ZgotmplZ`,
+			},
+		},
 		{
 			`<a title={{.}}>`,
 			[]string{
@@ -71,6 +88,7 @@ func TestTypedContent(t *testing.T) {
 				`a[href&#32;&#61;~&#32;&#34;//example.com&#34;]#foo`,
 				// Tags stripped, spaces escaped, entity not re-escaped.
 				`Hello,&#32;World&#32;&amp;tc!`,
+				`&#32;dir&#61;&#34;ltr&#34;`,
 				`c&#32;&amp;&amp;&#32;alert(&#34;Hello,&#32;World!&#34;);`,
 				`Hello,&#32;World&#32;&amp;&#32;O&#39;Reilly\x21`,
 				`greeting&#61;H%69&amp;addressee&#61;(World)`,
@@ -83,6 +101,7 @@ func TestTypedContent(t *testing.T) {
 				`a[href =~ &#34;//example.com&#34;]#foo`,
 				// Tags stripped, entity not re-escaped.
 				`Hello, World &amp;tc!`,
+				` dir=&#34;ltr&#34;`,
 				`c &amp;&amp; alert(&#34;Hello, World!&#34;);`,
 				`Hello, World &amp; O&#39;Reilly\x21`,
 				`greeting=H%69&amp;addressee=(World)`,
@@ -95,6 +114,7 @@ func TestTypedContent(t *testing.T) {
 				`a[href =~ &#34;//example.com&#34;]#foo`,
 				// Angle brackets escaped to prevent injection of close tags, entity not re-escaped.
 				`Hello, &lt;b&gt;World&lt;/b&gt; &amp;tc!`,
+				` dir=&#34;ltr&#34;`,
 				`c &amp;&amp; alert(&#34;Hello, World!&#34;);`,
 				`Hello, World &amp; O&#39;Reilly\x21`,
 				`greeting=H%69&amp;addressee=(World)`,
@@ -106,6 +126,7 @@ func TestTypedContent(t *testing.T) {
 				`"\u003cb\u003e \"foo%\" O'Reilly &bar;"`,
 				`"a[href =~ \"//example.com\"]#foo"`,
 				`"Hello, \u003cb\u003eWorld\u003c/b\u003e &amp;tc!"`,
+				`" dir=\"ltr\""`,
 				// Not escaped.
 				`c && alert("Hello, World!");`,
 				// Escape sequence not over-escaped.
@@ -119,6 +140,7 @@ func TestTypedContent(t *testing.T) {
 				`&#34;\u003cb\u003e \&#34;foo%\&#34; O&#39;Reilly &amp;bar;&#34;`,
 				`&#34;a[href =~ \&#34;//example.com\&#34;]#foo&#34;`,
 				`&#34;Hello, \u003cb\u003eWorld\u003c/b\u003e &amp;amp;tc!&#34;`,
+				`&#34; dir=\&#34;ltr\&#34;&#34;`,
 				// Not JS escaped but HTML escaped.
 				`c &amp;&amp; alert(&#34;Hello, World!&#34;);`,
 				// Escape sequence not over-escaped.
@@ -132,6 +154,7 @@ func TestTypedContent(t *testing.T) {
 				`\x3cb\x3e \x22foo%\x22 O\x27Reilly \x26bar;`,
 				`a[href =~ \x22\/\/example.com\x22]#foo`,
 				`Hello, \x3cb\x3eWorld\x3c\/b\x3e \x26amp;tc!`,
+				` dir=\x22ltr\x22`,
 				`c \x26\x26 alert(\x22Hello, World!\x22);`,
 				// Escape sequence not over-escaped.
 				`Hello, World \x26 O\x27Reilly\x21`,
@@ -144,6 +167,7 @@ func TestTypedContent(t *testing.T) {
 				`\x3cb\x3e \x22foo%\x22 O\x27Reilly \x26bar;`,
 				`a[href =~ \x22\/\/example.com\x22]#foo`,
 				`Hello, \x3cb\x3eWorld\x3c\/b\x3e \x26amp;tc!`,
+				` dir=\x22ltr\x22`,
 				`c \x26\x26 alert(\x22Hello, World!\x22);`,
 				// Escape sequence not over-escaped.
 				`Hello, World \x26 O\x27Reilly\x21`,
@@ -156,6 +180,7 @@ func TestTypedContent(t *testing.T) {
 				`%3cb%3e%20%22foo%25%22%20O%27Reilly%20%26bar%3b`,
 				`a%5bhref%20%3d~%20%22%2f%2fexample.com%22%5d%23foo`,
 				`Hello%2c%20%3cb%3eWorld%3c%2fb%3e%20%26amp%3btc%21`,
+				`%20dir%3d%22ltr%22`,
 				`c%20%26%26%20alert%28%22Hello%2c%20World%21%22%29%3b`,
 				`Hello%2c%20World%20%26%20O%27Reilly%5cx21`,
 				// Quotes and parens are escaped but %69 is not over-escaped. HTML escaping is done.
@@ -168,6 +193,7 @@ func TestTypedContent(t *testing.T) {
 				`%3cb%3e%20%22foo%25%22%20O%27Reilly%20%26bar%3b`,
 				`a%5bhref%20%3d~%20%22%2f%2fexample.com%22%5d%23foo`,
 				`Hello%2c%20%3cb%3eWorld%3c%2fb%3e%20%26amp%3btc%21`,
+				`%20dir%3d%22ltr%22`,
 				`c%20%26%26%20alert%28%22Hello%2c%20World%21%22%29%3b`,
 				`Hello%2c%20World%20%26%20O%27Reilly%5cx21`,
 				// Quotes and parens are escaped but %69 is not over-escaped. HTML escaping is not done.

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

@@ -20,6 +20,7 @@ type context struct {
 	delim   delim
 	urlPart urlPart
 	jsCtx   jsCtx
+	attr    attr
 	element element
 	err     *Error
 }
@@ -30,6 +31,7 @@ func (c context) eq(d context) bool {
 		c.delim == d.delim &&
 		c.urlPart == d.urlPart &&
 		c.jsCtx == d.jsCtx &&
+		c.attr == d.attr &&
 		c.element == d.element &&
 		c.err == d.err
 }
@@ -51,6 +53,9 @@ func (c context) mangle(templateName string) string {
 	if c.jsCtx != 0 {
 		s += "_" + c.jsCtx.String()
 	}
+	if c.attr != 0 {
+		s += "_" + c.attr.String()
+	}
 	if c.element != 0 {
 		s += "_" + c.element.String()
 	}
@@ -75,6 +80,15 @@ const (
 	stateText state = iota
 	// stateTag occurs before an HTML attribute or the end of a tag.
 	stateTag
+	// stateAttrName occurs inside an attribute name.
+	// It occurs between the ^'s in ` ^name^ = value`.
+	stateAttrName
+	// stateAfterName occurs after an attr name has ended but before any
+	// equals sign. It occurs between the ^'s in ` name^ ^= value`.
+	stateAfterName
+	// stateBeforeValue occurs after the equals sign but before the value.
+	// It occurs between the ^'s in ` name =^ ^value`.
+	stateBeforeValue
 	// stateComment occurs inside an <!-- HTML comment -->.
 	stateComment
 	// stateRCDATA occurs inside an RCDATA element (<textarea> or <title>)
@@ -120,6 +134,9 @@ const (
 var stateNames = [...]string{
 	stateText:        "stateText",
 	stateTag:         "stateTag",
+	stateAttrName:    "stateAttrName",
+	stateAfterName:   "stateAfterName",
+	stateBeforeValue: "stateBeforeValue",
 	stateComment:     "stateComment",
 	stateRCDATA:      "stateRCDATA",
 	stateAttr:        "stateAttr",
@@ -145,7 +162,7 @@ func (s state) String() string {
 	if int(s) < len(stateNames) {
 		return stateNames[s]
 	}
-	return fmt.Sprintf("illegal state %d", s)
+	return fmt.Sprintf("illegal state %d", int(s))
 }
 
 // delim is the delimiter that will end the current HTML attribute.
@@ -174,7 +191,7 @@ func (d delim) String() string {
 	if int(d) < len(delimNames) {
 		return delimNames[d]
 	}
-	return fmt.Sprintf("illegal delim %d", d)
+	return fmt.Sprintf("illegal delim %d", int(d))
 }
 
 // urlPart identifies a part in an RFC 3986 hierarchical URL to allow different
@@ -207,7 +224,7 @@ func (u urlPart) String() string {
 	if int(u) < len(urlPartNames) {
 		return urlPartNames[u]
 	}
-	return fmt.Sprintf("illegal urlPart %d", u)
+	return fmt.Sprintf("illegal urlPart %d", int(u))
 }
 
 // jsCtx determines whether a '/' starts a regular expression literal or a
@@ -232,7 +249,7 @@ func (c jsCtx) String() string {
 	case jsCtxUnknown:
 		return "jsCtxUnknown"
 	}
-	return fmt.Sprintf("illegal jsCtx %d", c)
+	return fmt.Sprintf("illegal jsCtx %d", int(c))
 }
 
 // element identifies the HTML element when inside a start tag or special body.
@@ -267,5 +284,33 @@ func (e element) String() string {
 	if int(e) < len(elementNames) {
 		return elementNames[e]
 	}
-	return fmt.Sprintf("illegal element %d", e)
+	return fmt.Sprintf("illegal element %d", int(e))
+}
+
+// attr identifies the most recent HTML attribute when inside a start tag.
+type attr uint8
+
+const (
+	// attrNone corresponds to a normal attribute or no attribute.
+	attrNone attr = iota
+	// attrScript corresponds to an event handler attribute.
+	attrScript
+	// attrStyle corresponds to the style attribute whose value is CSS.
+	attrStyle
+	// attrURL corresponds to an attribute whose value is a URL.
+	attrURL
+)
+
+var attrNames = [...]string{
+	attrNone:   "attrNone",
+	attrScript: "attrScript",
+	attrStyle:  "attrStyle",
+	attrURL:    "attrURL",
+}
+
+func (a attr) String() string {
+	if int(a) < len(attrNames) {
+		return attrNames[a]
+	}
+	return fmt.Sprintf("illegal attr %d", int(a))
 }

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

@@ -66,6 +66,7 @@ var funcMap = template.FuncMap{
 	"exp_template_html_attrescaper":     attrEscaper,
 	"exp_template_html_cssescaper":      cssEscaper,
 	"exp_template_html_cssvaluefilter":  cssValueFilter,
+	"exp_template_html_htmlnamefilter":  htmlNameFilter,
 	"exp_template_html_htmlescaper":     htmlEscaper,
 	"exp_template_html_jsregexpescaper": jsRegexpEscaper,
 	"exp_template_html_jsstrescaper":    jsStrEscaper,
@@ -151,8 +152,11 @@ func (e *escaper) escape(c context, n parse.Node) context {
 
 // escapeAction escapes an action template node.
 func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
+	c = nudge(c)
 	s := make([]string, 0, 3)
 	switch c.state {
+	case stateError:
+		return c
 	case stateURL, stateCSSDqStr, stateCSSSqStr, stateCSSDqURL, stateCSSSqURL, stateCSSURL:
 		switch c.urlPart {
 		case urlPartNone:
@@ -194,6 +198,13 @@ func (e *escaper) escapeAction(c context, n *parse.ActionNode) context {
 		s = append(s, "exp_template_html_htmlescaper")
 	case stateRCDATA:
 		s = append(s, "exp_template_html_rcdataescaper")
+	case stateAttr:
+		// Handled below in delim check.
+	case stateAttrName, stateTag:
+		c.state = stateAttrName
+		s = append(s, "exp_template_html_htmlnamefilter")
+	default:
+		panic("unexpected state " + c.state.String())
 	}
 	switch c.delim {
 	case delimNone:
@@ -289,6 +300,33 @@ func newIdentCmd(identifier string) *parse.CommandNode {
 	}
 }
 
+// nudge returns the context that would result from following empty string
+// transitions from the input context.
+// For example, parsing:
+//     `<a href=`
+// will end in context{stateBeforeValue, attrURL}, but parsing one extra rune:
+//     `<a href=x`
+// will end in context{stateURL, delimSpaceOrTagEnd, ...}.
+// There are two transitions that happen when the 'x' is seen:
+// (1) Transition from a before-value state to a start-of-value state without
+//     consuming any character.
+// (2) Consume 'x' and transition past the first value character.
+// In this case, nudging produces the context after (1) happens.
+func nudge(c context) context {
+	switch c.state {
+	case stateTag:
+		// In `<foo {{.}}`, the action should emit an attribute.
+		c.state = stateAttrName
+	case stateBeforeValue:
+		// In `<foo bar={{.}}`, the action is an undelimited value.
+		c.state, c.delim, c.attr = attrStartStates[c.attr], delimSpaceOrTagEnd, attrNone
+	case stateAfterName:
+		// In `<foo bar {{.}}`, the action is an attribute name.
+		c.state, c.attr = stateAttrName, attrNone
+	}
+	return c
+}
+
 // join joins the two contexts of a branch template node. The result is an
 // error context if either of the input contexts are error contexts, or if the
 // the input contexts differ.
@@ -319,6 +357,17 @@ func join(a, b context, line int, nodeName string) context {
 		return c
 	}
 
+	// Allow a nudged context to join with an unnudged one.
+	// This means that
+	//   <p title={{if .C}}{{.}}{{end}}
+	// ends in an unquoted value state even though the else branch
+	// ends in stateBeforeValue.
+	if c, d := nudge(a), nudge(b); !(c.eq(a) && d.eq(b)) {
+		if e := join(c, d, line, nodeName); e.state != stateError {
+			return e
+		}
+	}
+
 	return context{
 		state: stateError,
 		err:   errorf(ErrBranchEnd, line, "{{%s}} branches end in different contexts: %v, %v", nodeName, a, b),

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

@@ -411,6 +411,51 @@ func TestEscape(t *testing.T) {
 			`<textarea><{{"/textarea "}}...</textarea>`,
 			`<textarea>&lt;/textarea ...</textarea>`,
 		},
+		{
+			"optional attrs",
+			`<img class="{{"iconClass"}}"` +
+				`{{if .T}} id="{{"<iconId>"}}"{{end}}` +
+				// Double quotes inside if/else.
+				` src=` +
+				`{{if .T}}"?{{"<iconPath>"}}"` +
+				`{{else}}"images/cleardot.gif"{{end}}` +
+				// Missing space before title, but it is not a
+				// part of the src attribute.
+				`{{if .T}}title="{{"<title>"}}"{{end}}` +
+				// Quotes outside if/else.
+				` alt="` +
+				`{{if .T}}{{"<alt>"}}` +
+				`{{else}}{{if .F}}{{"<title>"}}{{end}}` +
+				`{{end}}"` +
+				`>`,
+			`<img class="iconClass" id="&lt;iconId&gt;" src="?%3ciconPath%3e"title="&lt;title&gt;" alt="&lt;alt&gt;">`,
+		},
+		{
+			"conditional valueless attr name",
+			`<input{{if .T}} checked{{end}} name=n>`,
+			`<input checked name=n>`,
+		},
+		{
+			"conditional dynamic valueless attr name 1",
+			`<input{{if .T}} {{"checked"}}{{end}} name=n>`,
+			`<input checked name=n>`,
+		},
+		{
+			"conditional dynamic valueless attr name 2",
+			`<input {{if .T}}{{"checked"}} {{end}}name=n>`,
+			`<input checked name=n>`,
+		},
+		{
+			"dynamic attribute name",
+			`<img on{{"load"}}="alert({{"loaded"}})">`,
+			// Treated as JS since quotes are inserted.
+			`<img onload="alert(&#34;loaded&#34;)">`,
+		},
+		{
+			"dynamic element name",
+			`<h{{3}}><table><t{{"head"}}>...</h{{3}}>`,
+			`<h3><table><thead>...</h3>`,
+		},
 	}
 
 	for _, test := range tests {
@@ -780,9 +825,25 @@ func TestEscapeText(t *testing.T) {
 			`<a>`,
 			context{state: stateText},
 		},
+		{
+			`<a href`,
+			context{state: stateAttrName, attr: attrURL},
+		},
+		{
+			`<a on`,
+			context{state: stateAttrName, attr: attrScript},
+		},
+		{
+			`<a href `,
+			context{state: stateAfterName, attr: attrURL},
+		},
+		{
+			`<a style  =  `,
+			context{state: stateBeforeValue, attr: attrStyle},
+		},
 		{
 			`<a href=`,
-			context{state: stateURL, delim: delimSpaceOrTagEnd},
+			context{state: stateBeforeValue, attr: attrURL},
 		},
 		{
 			`<a href=x`,

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

@@ -205,3 +205,22 @@ func stripTags(html string) string {
 	}
 	return b.String()
 }
+
+// htmlNameFilter accepts valid parts of an HTML attribute or tag name or
+// a known-safe HTML attribute.
+func htmlNameFilter(args ...interface{}) string {
+	s, t := stringify(args...)
+	if t == contentTypeHTMLAttr {
+		return s
+	}
+	for _, r := range s {
+		switch {
+		case '0' <= r && r <= '9':
+		case 'A' <= r && r <= 'Z':
+		case 'a' <= r && r <= 'z':
+		default:
+			return filterFailsafe
+		}
+	}
+	return s
+}

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

@@ -18,6 +18,9 @@ import (
 var transitionFunc = [...]func(context, []byte) (context, []byte){
 	stateText:        tText,
 	stateTag:         tTag,
+	stateAttrName:    tAttrName,
+	stateAfterName:   tAfterName,
+	stateBeforeValue: tBeforeValue,
 	stateComment:     tComment,
 	stateRCDATA:      tSpecialTagEnd,
 	stateAttr:        tAttr,
@@ -79,54 +82,90 @@ var elementContentType = [...]state{
 // tTag is the context transition function for the tag state.
 func tTag(c context, s []byte) (context, []byte) {
 	// Find the attribute name.
-	attrStart := eatWhiteSpace(s, 0)
-	i, err := eatAttrName(s, attrStart)
-	if err != nil {
-		return context{
-			state: stateError,
-			err:   err,
-		}, nil
-	}
+	i := eatWhiteSpace(s, 0)
 	if i == len(s) {
 		return c, nil
 	}
-	state := stateAttr
-	canonAttrName := strings.ToLower(string(s[attrStart:i]))
-	if urlAttr[canonAttrName] {
-		state = stateURL
-	} else if strings.HasPrefix(canonAttrName, "on") {
-		state = stateJS
-	} else if canonAttrName == "style" {
-		state = stateCSS
+	if s[i] == '>' {
+		return context{
+			state:   elementContentType[c.element],
+			element: c.element,
+		}, s[i+1:]
+	}
+	j, err := eatAttrName(s, i)
+	if err != nil {
+		return context{state: stateError, err: err}, nil
+	}
+	state, attr := stateTag, attrNone
+	if i != j {
+		canonAttrName := strings.ToLower(string(s[i:j]))
+		if urlAttr[canonAttrName] {
+			attr = attrURL
+		} else if strings.HasPrefix(canonAttrName, "on") {
+			attr = attrScript
+		} else if canonAttrName == "style" {
+			attr = attrStyle
+		}
+		if j == len(s) {
+			state = stateAttrName
+		} else {
+			state = stateAfterName
+		}
+	}
+	return context{state: state, element: c.element, attr: attr}, s[j:]
+}
+
+// tAttrName is the context transition function for stateAttrName.
+func tAttrName(c context, s []byte) (context, []byte) {
+	i, err := eatAttrName(s, 0)
+	if err != nil {
+		return context{state: stateError, err: err}, nil
+	} else if i == len(s) {
+		return c, nil
 	}
+	c.state = stateAfterName
+	return c, s[i:]
+}
 
+// tAfterName is the context transition function for stateAfterName.
+func tAfterName(c context, s []byte) (context, []byte) {
 	// Look for the start of the value.
-	i = eatWhiteSpace(s, i)
+	i := eatWhiteSpace(s, 0)
 	if i == len(s) {
-		return c, s[i:]
-	}
-	if s[i] == '>' {
-		state = elementContentType[c.element]
-		return context{state: state, element: c.element}, s[i+1:]
+		return c, nil
 	} else if s[i] != '=' {
-		// Possible due to a valueless attribute or '/' in "<input />".
+		// Occurs due to tag ending '>', and valueless attribute.
+		c.state = stateTag
 		return c, s[i:]
 	}
+	c.state = stateBeforeValue
 	// Consume the "=".
-	i = eatWhiteSpace(s, i+1)
+	return c, s[i+1:]
+}
 
+var attrStartStates = [...]state{
+	attrNone:   stateAttr,
+	attrScript: stateJS,
+	attrStyle:  stateCSS,
+	attrURL:    stateURL,
+}
+
+// tBeforeValue is the context transition function for stateBeforeValue.
+func tBeforeValue(c context, s []byte) (context, []byte) {
+	i := eatWhiteSpace(s, 0)
+	if i == len(s) {
+		return c, nil
+	}
 	// Find the attribute delimiter.
 	delim := delimSpaceOrTagEnd
-	if i < len(s) {
-		switch s[i] {
-		case '\'':
-			delim, i = delimSingleQuote, i+1
-		case '"':
-			delim, i = delimDoubleQuote, i+1
-		}
+	switch s[i] {
+	case '\'':
+		delim, i = delimSingleQuote, i+1
+	case '"':
+		delim, i = delimDoubleQuote, i+1
 	}
-
-	return context{state: state, delim: delim, element: c.element}, s[i:]
+	c.state, c.delim, c.attr = attrStartStates[c.attr], delim, attrNone
+	return c, s[i:]
 }
 
 // tComment is the context transition function for stateComment.