[chore/bugfix] Switch markdown from blackfriday to goldmark (#1267)

Co-authored-by: Autumn! <autumnull@posteo.net>

vendor/github.com/yuin/goldmark/renderer/html/html.go

@@ -0,0 +1,931 @@
+package html
+
+import (
+	"bytes"
+	"fmt"
+	"strconv"
+	"unicode/utf8"
+
+	"github.com/yuin/goldmark/ast"
+	"github.com/yuin/goldmark/renderer"
+	"github.com/yuin/goldmark/util"
+)
+
+// A Config struct has configurations for the HTML based renderers.
+type Config struct {
+	Writer              Writer
+	HardWraps           bool
+	EastAsianLineBreaks bool
+	XHTML               bool
+	Unsafe              bool
+}
+
+// NewConfig returns a new Config with defaults.
+func NewConfig() Config {
+	return Config{
+		Writer:              DefaultWriter,
+		HardWraps:           false,
+		EastAsianLineBreaks: false,
+		XHTML:               false,
+		Unsafe:              false,
+	}
+}
+
+// SetOption implements renderer.NodeRenderer.SetOption.
+func (c *Config) SetOption(name renderer.OptionName, value interface{}) {
+	switch name {
+	case optHardWraps:
+		c.HardWraps = value.(bool)
+	case optEastAsianLineBreaks:
+		c.EastAsianLineBreaks = value.(bool)
+	case optXHTML:
+		c.XHTML = value.(bool)
+	case optUnsafe:
+		c.Unsafe = value.(bool)
+	case optTextWriter:
+		c.Writer = value.(Writer)
+	}
+}
+
+// An Option interface sets options for HTML based renderers.
+type Option interface {
+	SetHTMLOption(*Config)
+}
+
+// TextWriter is an option name used in WithWriter.
+const optTextWriter renderer.OptionName = "Writer"
+
+type withWriter struct {
+	value Writer
+}
+
+func (o *withWriter) SetConfig(c *renderer.Config) {
+	c.Options[optTextWriter] = o.value
+}
+
+func (o *withWriter) SetHTMLOption(c *Config) {
+	c.Writer = o.value
+}
+
+// WithWriter is a functional option that allow you to set the given writer to
+// the renderer.
+func WithWriter(writer Writer) interface {
+	renderer.Option
+	Option
+} {
+	return &withWriter{writer}
+}
+
+// HardWraps is an option name used in WithHardWraps.
+const optHardWraps renderer.OptionName = "HardWraps"
+
+type withHardWraps struct {
+}
+
+func (o *withHardWraps) SetConfig(c *renderer.Config) {
+	c.Options[optHardWraps] = true
+}
+
+func (o *withHardWraps) SetHTMLOption(c *Config) {
+	c.HardWraps = true
+}
+
+// WithHardWraps is a functional option that indicates whether softline breaks
+// should be rendered as '<br>'.
+func WithHardWraps() interface {
+	renderer.Option
+	Option
+} {
+	return &withHardWraps{}
+}
+
+// EastAsianLineBreaks is an option name used in WithEastAsianLineBreaks.
+const optEastAsianLineBreaks renderer.OptionName = "EastAsianLineBreaks"
+
+type withEastAsianLineBreaks struct {
+}
+
+func (o *withEastAsianLineBreaks) SetConfig(c *renderer.Config) {
+	c.Options[optEastAsianLineBreaks] = true
+}
+
+func (o *withEastAsianLineBreaks) SetHTMLOption(c *Config) {
+	c.EastAsianLineBreaks = true
+}
+
+// WithEastAsianLineBreaks is a functional option that indicates whether softline breaks
+// between east asian wide characters should be ignored.
+func WithEastAsianLineBreaks() interface {
+	renderer.Option
+	Option
+} {
+	return &withEastAsianLineBreaks{}
+}
+
+// XHTML is an option name used in WithXHTML.
+const optXHTML renderer.OptionName = "XHTML"
+
+type withXHTML struct {
+}
+
+func (o *withXHTML) SetConfig(c *renderer.Config) {
+	c.Options[optXHTML] = true
+}
+
+func (o *withXHTML) SetHTMLOption(c *Config) {
+	c.XHTML = true
+}
+
+// WithXHTML is a functional option indicates that nodes should be rendered in
+// xhtml instead of HTML5.
+func WithXHTML() interface {
+	Option
+	renderer.Option
+} {
+	return &withXHTML{}
+}
+
+// Unsafe is an option name used in WithUnsafe.
+const optUnsafe renderer.OptionName = "Unsafe"
+
+type withUnsafe struct {
+}
+
+func (o *withUnsafe) SetConfig(c *renderer.Config) {
+	c.Options[optUnsafe] = true
+}
+
+func (o *withUnsafe) SetHTMLOption(c *Config) {
+	c.Unsafe = true
+}
+
+// WithUnsafe is a functional option that renders dangerous contents
+// (raw htmls and potentially dangerous links) as it is.
+func WithUnsafe() interface {
+	renderer.Option
+	Option
+} {
+	return &withUnsafe{}
+}
+
+// A Renderer struct is an implementation of renderer.NodeRenderer that renders
+// nodes as (X)HTML.
+type Renderer struct {
+	Config
+}
+
+// NewRenderer returns a new Renderer with given options.
+func NewRenderer(opts ...Option) renderer.NodeRenderer {
+	r := &Renderer{
+		Config: NewConfig(),
+	}
+
+	for _, opt := range opts {
+		opt.SetHTMLOption(&r.Config)
+	}
+	return r
+}
+
+// RegisterFuncs implements NodeRenderer.RegisterFuncs .
+func (r *Renderer) RegisterFuncs(reg renderer.NodeRendererFuncRegisterer) {
+	// blocks
+
+	reg.Register(ast.KindDocument, r.renderDocument)
+	reg.Register(ast.KindHeading, r.renderHeading)
+	reg.Register(ast.KindBlockquote, r.renderBlockquote)
+	reg.Register(ast.KindCodeBlock, r.renderCodeBlock)
+	reg.Register(ast.KindFencedCodeBlock, r.renderFencedCodeBlock)
+	reg.Register(ast.KindHTMLBlock, r.renderHTMLBlock)
+	reg.Register(ast.KindList, r.renderList)
+	reg.Register(ast.KindListItem, r.renderListItem)
+	reg.Register(ast.KindParagraph, r.renderParagraph)
+	reg.Register(ast.KindTextBlock, r.renderTextBlock)
+	reg.Register(ast.KindThematicBreak, r.renderThematicBreak)
+
+	// inlines
+
+	reg.Register(ast.KindAutoLink, r.renderAutoLink)
+	reg.Register(ast.KindCodeSpan, r.renderCodeSpan)
+	reg.Register(ast.KindEmphasis, r.renderEmphasis)
+	reg.Register(ast.KindImage, r.renderImage)
+	reg.Register(ast.KindLink, r.renderLink)
+	reg.Register(ast.KindRawHTML, r.renderRawHTML)
+	reg.Register(ast.KindText, r.renderText)
+	reg.Register(ast.KindString, r.renderString)
+}
+
+func (r *Renderer) writeLines(w util.BufWriter, source []byte, n ast.Node) {
+	l := n.Lines().Len()
+	for i := 0; i < l; i++ {
+		line := n.Lines().At(i)
+		r.Writer.RawWrite(w, line.Value(source))
+	}
+}
+
+// GlobalAttributeFilter defines attribute names which any elements can have.
+var GlobalAttributeFilter = util.NewBytesFilter(
+	[]byte("accesskey"),
+	[]byte("autocapitalize"),
+	[]byte("autofocus"),
+	[]byte("class"),
+	[]byte("contenteditable"),
+	[]byte("dir"),
+	[]byte("draggable"),
+	[]byte("enterkeyhint"),
+	[]byte("hidden"),
+	[]byte("id"),
+	[]byte("inert"),
+	[]byte("inputmode"),
+	[]byte("is"),
+	[]byte("itemid"),
+	[]byte("itemprop"),
+	[]byte("itemref"),
+	[]byte("itemscope"),
+	[]byte("itemtype"),
+	[]byte("lang"),
+	[]byte("part"),
+	[]byte("slot"),
+	[]byte("spellcheck"),
+	[]byte("style"),
+	[]byte("tabindex"),
+	[]byte("title"),
+	[]byte("translate"),
+)
+
+func (r *Renderer) renderDocument(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	// nothing to do
+	return ast.WalkContinue, nil
+}
+
+// HeadingAttributeFilter defines attribute names which heading elements can have
+var HeadingAttributeFilter = GlobalAttributeFilter
+
+func (r *Renderer) renderHeading(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	n := node.(*ast.Heading)
+	if entering {
+		_, _ = w.WriteString("<h")
+		_ = w.WriteByte("0123456"[n.Level])
+		if n.Attributes() != nil {
+			RenderAttributes(w, node, HeadingAttributeFilter)
+		}
+		_ = w.WriteByte('>')
+	} else {
+		_, _ = w.WriteString("</h")
+		_ = w.WriteByte("0123456"[n.Level])
+		_, _ = w.WriteString(">\n")
+	}
+	return ast.WalkContinue, nil
+}
+
+// BlockquoteAttributeFilter defines attribute names which blockquote elements can have
+var BlockquoteAttributeFilter = GlobalAttributeFilter.Extend(
+	[]byte("cite"),
+)
+
+func (r *Renderer) renderBlockquote(w util.BufWriter, source []byte, n ast.Node, entering bool) (ast.WalkStatus, error) {
+	if entering {
+		if n.Attributes() != nil {
+			_, _ = w.WriteString("<blockquote")
+			RenderAttributes(w, n, BlockquoteAttributeFilter)
+			_ = w.WriteByte('>')
+		} else {
+			_, _ = w.WriteString("<blockquote>\n")
+		}
+	} else {
+		_, _ = w.WriteString("</blockquote>\n")
+	}
+	return ast.WalkContinue, nil
+}
+
+func (r *Renderer) renderCodeBlock(w util.BufWriter, source []byte, n ast.Node, entering bool) (ast.WalkStatus, error) {
+	if entering {
+		_, _ = w.WriteString("<pre><code>")
+		r.writeLines(w, source, n)
+	} else {
+		_, _ = w.WriteString("</code></pre>\n")
+	}
+	return ast.WalkContinue, nil
+}
+
+func (r *Renderer) renderFencedCodeBlock(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	n := node.(*ast.FencedCodeBlock)
+	if entering {
+		_, _ = w.WriteString("<pre><code")
+		language := n.Language(source)
+		if language != nil {
+			_, _ = w.WriteString(" class=\"language-")
+			r.Writer.Write(w, language)
+			_, _ = w.WriteString("\"")
+		}
+		_ = w.WriteByte('>')
+		r.writeLines(w, source, n)
+	} else {
+		_, _ = w.WriteString("</code></pre>\n")
+	}
+	return ast.WalkContinue, nil
+}
+
+func (r *Renderer) renderHTMLBlock(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	n := node.(*ast.HTMLBlock)
+	if entering {
+		if r.Unsafe {
+			l := n.Lines().Len()
+			for i := 0; i < l; i++ {
+				line := n.Lines().At(i)
+				r.Writer.SecureWrite(w, line.Value(source))
+			}
+		} else {
+			_, _ = w.WriteString("<!-- raw HTML omitted -->\n")
+		}
+	} else {
+		if n.HasClosure() {
+			if r.Unsafe {
+				closure := n.ClosureLine
+				r.Writer.SecureWrite(w, closure.Value(source))
+			} else {
+				_, _ = w.WriteString("<!-- raw HTML omitted -->\n")
+			}
+		}
+	}
+	return ast.WalkContinue, nil
+}
+
+// ListAttributeFilter defines attribute names which list elements can have.
+var ListAttributeFilter = GlobalAttributeFilter.Extend(
+	[]byte("start"),
+	[]byte("reversed"),
+	[]byte("type"),
+)
+
+func (r *Renderer) renderList(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	n := node.(*ast.List)
+	tag := "ul"
+	if n.IsOrdered() {
+		tag = "ol"
+	}
+	if entering {
+		_ = w.WriteByte('<')
+		_, _ = w.WriteString(tag)
+		if n.IsOrdered() && n.Start != 1 {
+			fmt.Fprintf(w, " start=\"%d\"", n.Start)
+		}
+		if n.Attributes() != nil {
+			RenderAttributes(w, n, ListAttributeFilter)
+		}
+		_, _ = w.WriteString(">\n")
+	} else {
+		_, _ = w.WriteString("</")
+		_, _ = w.WriteString(tag)
+		_, _ = w.WriteString(">\n")
+	}
+	return ast.WalkContinue, nil
+}
+
+// ListItemAttributeFilter defines attribute names which list item elements can have.
+var ListItemAttributeFilter = GlobalAttributeFilter.Extend(
+	[]byte("value"),
+)
+
+func (r *Renderer) renderListItem(w util.BufWriter, source []byte, n ast.Node, entering bool) (ast.WalkStatus, error) {
+	if entering {
+		if n.Attributes() != nil {
+			_, _ = w.WriteString("<li")
+			RenderAttributes(w, n, ListItemAttributeFilter)
+			_ = w.WriteByte('>')
+		} else {
+			_, _ = w.WriteString("<li>")
+		}
+		fc := n.FirstChild()
+		if fc != nil {
+			if _, ok := fc.(*ast.TextBlock); !ok {
+				_ = w.WriteByte('\n')
+			}
+		}
+	} else {
+		_, _ = w.WriteString("</li>\n")
+	}
+	return ast.WalkContinue, nil
+}
+
+// ParagraphAttributeFilter defines attribute names which paragraph elements can have.
+var ParagraphAttributeFilter = GlobalAttributeFilter
+
+func (r *Renderer) renderParagraph(w util.BufWriter, source []byte, n ast.Node, entering bool) (ast.WalkStatus, error) {
+	if entering {
+		if n.Attributes() != nil {
+			_, _ = w.WriteString("<p")
+			RenderAttributes(w, n, ParagraphAttributeFilter)
+			_ = w.WriteByte('>')
+		} else {
+			_, _ = w.WriteString("<p>")
+		}
+	} else {
+		_, _ = w.WriteString("</p>\n")
+	}
+	return ast.WalkContinue, nil
+}
+
+func (r *Renderer) renderTextBlock(w util.BufWriter, source []byte, n ast.Node, entering bool) (ast.WalkStatus, error) {
+	if !entering {
+		if _, ok := n.NextSibling().(ast.Node); ok && n.FirstChild() != nil {
+			_ = w.WriteByte('\n')
+		}
+	}
+	return ast.WalkContinue, nil
+}
+
+// ThematicAttributeFilter defines attribute names which hr elements can have.
+var ThematicAttributeFilter = GlobalAttributeFilter.Extend(
+	[]byte("align"),   // [Deprecated]
+	[]byte("color"),   // [Not Standardized]
+	[]byte("noshade"), // [Deprecated]
+	[]byte("size"),    // [Deprecated]
+	[]byte("width"),   // [Deprecated]
+)
+
+func (r *Renderer) renderThematicBreak(w util.BufWriter, source []byte, n ast.Node, entering bool) (ast.WalkStatus, error) {
+	if !entering {
+		return ast.WalkContinue, nil
+	}
+	_, _ = w.WriteString("<hr")
+	if n.Attributes() != nil {
+		RenderAttributes(w, n, ThematicAttributeFilter)
+	}
+	if r.XHTML {
+		_, _ = w.WriteString(" />\n")
+	} else {
+		_, _ = w.WriteString(">\n")
+	}
+	return ast.WalkContinue, nil
+}
+
+// LinkAttributeFilter defines attribute names which link elements can have.
+var LinkAttributeFilter = GlobalAttributeFilter.Extend(
+	[]byte("download"),
+	// []byte("href"),
+	[]byte("hreflang"),
+	[]byte("media"),
+	[]byte("ping"),
+	[]byte("referrerpolicy"),
+	[]byte("rel"),
+	[]byte("shape"),
+	[]byte("target"),
+)
+
+func (r *Renderer) renderAutoLink(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	n := node.(*ast.AutoLink)
+	if !entering {
+		return ast.WalkContinue, nil
+	}
+	_, _ = w.WriteString(`<a href="`)
+	url := n.URL(source)
+	label := n.Label(source)
+	if n.AutoLinkType == ast.AutoLinkEmail && !bytes.HasPrefix(bytes.ToLower(url), []byte("mailto:")) {
+		_, _ = w.WriteString("mailto:")
+	}
+	_, _ = w.Write(util.EscapeHTML(util.URLEscape(url, false)))
+	if n.Attributes() != nil {
+		_ = w.WriteByte('"')
+		RenderAttributes(w, n, LinkAttributeFilter)
+		_ = w.WriteByte('>')
+	} else {
+		_, _ = w.WriteString(`">`)
+	}
+	_, _ = w.Write(util.EscapeHTML(label))
+	_, _ = w.WriteString(`</a>`)
+	return ast.WalkContinue, nil
+}
+
+// CodeAttributeFilter defines attribute names which code elements can have.
+var CodeAttributeFilter = GlobalAttributeFilter
+
+func (r *Renderer) renderCodeSpan(w util.BufWriter, source []byte, n ast.Node, entering bool) (ast.WalkStatus, error) {
+	if entering {
+		if n.Attributes() != nil {
+			_, _ = w.WriteString("<code")
+			RenderAttributes(w, n, CodeAttributeFilter)
+			_ = w.WriteByte('>')
+		} else {
+			_, _ = w.WriteString("<code>")
+		}
+		for c := n.FirstChild(); c != nil; c = c.NextSibling() {
+			segment := c.(*ast.Text).Segment
+			value := segment.Value(source)
+			if bytes.HasSuffix(value, []byte("\n")) {
+				r.Writer.RawWrite(w, value[:len(value)-1])
+				r.Writer.RawWrite(w, []byte(" "))
+			} else {
+				r.Writer.RawWrite(w, value)
+			}
+		}
+		return ast.WalkSkipChildren, nil
+	}
+	_, _ = w.WriteString("</code>")
+	return ast.WalkContinue, nil
+}
+
+// EmphasisAttributeFilter defines attribute names which emphasis elements can have.
+var EmphasisAttributeFilter = GlobalAttributeFilter
+
+func (r *Renderer) renderEmphasis(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	n := node.(*ast.Emphasis)
+	tag := "em"
+	if n.Level == 2 {
+		tag = "strong"
+	}
+	if entering {
+		_ = w.WriteByte('<')
+		_, _ = w.WriteString(tag)
+		if n.Attributes() != nil {
+			RenderAttributes(w, n, EmphasisAttributeFilter)
+		}
+		_ = w.WriteByte('>')
+	} else {
+		_, _ = w.WriteString("</")
+		_, _ = w.WriteString(tag)
+		_ = w.WriteByte('>')
+	}
+	return ast.WalkContinue, nil
+}
+
+func (r *Renderer) renderLink(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	n := node.(*ast.Link)
+	if entering {
+		_, _ = w.WriteString("<a href=\"")
+		if r.Unsafe || !IsDangerousURL(n.Destination) {
+			_, _ = w.Write(util.EscapeHTML(util.URLEscape(n.Destination, true)))
+		}
+		_ = w.WriteByte('"')
+		if n.Title != nil {
+			_, _ = w.WriteString(` title="`)
+			r.Writer.Write(w, n.Title)
+			_ = w.WriteByte('"')
+		}
+		if n.Attributes() != nil {
+			RenderAttributes(w, n, LinkAttributeFilter)
+		}
+		_ = w.WriteByte('>')
+	} else {
+		_, _ = w.WriteString("</a>")
+	}
+	return ast.WalkContinue, nil
+}
+
+// ImageAttributeFilter defines attribute names which image elements can have.
+var ImageAttributeFilter = GlobalAttributeFilter.Extend(
+	[]byte("align"),
+	[]byte("border"),
+	[]byte("crossorigin"),
+	[]byte("decoding"),
+	[]byte("height"),
+	[]byte("importance"),
+	[]byte("intrinsicsize"),
+	[]byte("ismap"),
+	[]byte("loading"),
+	[]byte("referrerpolicy"),
+	[]byte("sizes"),
+	[]byte("srcset"),
+	[]byte("usemap"),
+	[]byte("width"),
+)
+
+func (r *Renderer) renderImage(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	if !entering {
+		return ast.WalkContinue, nil
+	}
+	n := node.(*ast.Image)
+	_, _ = w.WriteString("<img src=\"")
+	if r.Unsafe || !IsDangerousURL(n.Destination) {
+		_, _ = w.Write(util.EscapeHTML(util.URLEscape(n.Destination, true)))
+	}
+	_, _ = w.WriteString(`" alt="`)
+	_, _ = w.Write(nodeToHTMLText(n, source))
+	_ = w.WriteByte('"')
+	if n.Title != nil {
+		_, _ = w.WriteString(` title="`)
+		r.Writer.Write(w, n.Title)
+		_ = w.WriteByte('"')
+	}
+	if n.Attributes() != nil {
+		RenderAttributes(w, n, ImageAttributeFilter)
+	}
+	if r.XHTML {
+		_, _ = w.WriteString(" />")
+	} else {
+		_, _ = w.WriteString(">")
+	}
+	return ast.WalkSkipChildren, nil
+}
+
+func (r *Renderer) renderRawHTML(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	if !entering {
+		return ast.WalkSkipChildren, nil
+	}
+	if r.Unsafe {
+		n := node.(*ast.RawHTML)
+		l := n.Segments.Len()
+		for i := 0; i < l; i++ {
+			segment := n.Segments.At(i)
+			_, _ = w.Write(segment.Value(source))
+		}
+		return ast.WalkSkipChildren, nil
+	}
+	_, _ = w.WriteString("<!-- raw HTML omitted -->")
+	return ast.WalkSkipChildren, nil
+}
+
+func (r *Renderer) renderText(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	if !entering {
+		return ast.WalkContinue, nil
+	}
+	n := node.(*ast.Text)
+	segment := n.Segment
+	if n.IsRaw() {
+		r.Writer.RawWrite(w, segment.Value(source))
+	} else {
+		value := segment.Value(source)
+		r.Writer.Write(w, value)
+		if n.HardLineBreak() || (n.SoftLineBreak() && r.HardWraps) {
+			if r.XHTML {
+				_, _ = w.WriteString("<br />\n")
+			} else {
+				_, _ = w.WriteString("<br>\n")
+			}
+		} else if n.SoftLineBreak() {
+			if r.EastAsianLineBreaks && len(value) != 0 {
+				sibling := node.NextSibling()
+				if sibling != nil && sibling.Kind() == ast.KindText {
+					if siblingText := sibling.(*ast.Text).Text(source); len(siblingText) != 0 {
+						thisLastRune := util.ToRune(value, len(value)-1)
+						siblingFirstRune, _ := utf8.DecodeRune(siblingText)
+						if !(util.IsEastAsianWideRune(thisLastRune) &&
+							util.IsEastAsianWideRune(siblingFirstRune)) {
+							_ = w.WriteByte('\n')
+						}
+					}
+				}
+			} else {
+				_ = w.WriteByte('\n')
+			}
+		}
+	}
+	return ast.WalkContinue, nil
+}
+
+func (r *Renderer) renderString(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
+	if !entering {
+		return ast.WalkContinue, nil
+	}
+	n := node.(*ast.String)
+	if n.IsCode() {
+		_, _ = w.Write(n.Value)
+	} else {
+		if n.IsRaw() {
+			r.Writer.RawWrite(w, n.Value)
+		} else {
+			r.Writer.Write(w, n.Value)
+		}
+	}
+	return ast.WalkContinue, nil
+}
+
+var dataPrefix = []byte("data-")
+
+// RenderAttributes renders given node's attributes.
+// You can specify attribute names to render by the filter.
+// If filter is nil, RenderAttributes renders all attributes.
+func RenderAttributes(w util.BufWriter, node ast.Node, filter util.BytesFilter) {
+	for _, attr := range node.Attributes() {
+		if filter != nil && !filter.Contains(attr.Name) {
+			if !bytes.HasPrefix(attr.Name, dataPrefix) {
+				continue
+			}
+		}
+		_, _ = w.WriteString(" ")
+		_, _ = w.Write(attr.Name)
+		_, _ = w.WriteString(`="`)
+		// TODO: convert numeric values to strings
+		_, _ = w.Write(util.EscapeHTML(attr.Value.([]byte)))
+		_ = w.WriteByte('"')
+	}
+}
+
+// A Writer interface writes textual contents to a writer.
+type Writer interface {
+	// Write writes the given source to writer with resolving references and unescaping
+	// backslash escaped characters.
+	Write(writer util.BufWriter, source []byte)
+
+	// RawWrite writes the given source to writer without resolving references and
+	// unescaping backslash escaped characters.
+	RawWrite(writer util.BufWriter, source []byte)
+
+	// SecureWrite writes the given source to writer with replacing insecure characters.
+	SecureWrite(writer util.BufWriter, source []byte)
+}
+
+var replacementCharacter = []byte("\ufffd")
+
+// A WriterConfig struct has configurations for the HTML based writers.
+type WriterConfig struct {
+	// EscapedSpace is an option that indicates that a '\' escaped half-space(0x20) should not be rendered.
+	EscapedSpace bool
+}
+
+// A WriterOption interface sets options for HTML based writers.
+type WriterOption func(*WriterConfig)
+
+// WithEscapedSpace is a WriterOption indicates that a '\' escaped half-space(0x20) should not be rendered.
+func WithEscapedSpace() WriterOption {
+	return func(c *WriterConfig) {
+		c.EscapedSpace = true
+	}
+}
+
+type defaultWriter struct {
+	WriterConfig
+}
+
+// NewWriter returns a new Writer.
+func NewWriter(opts ...WriterOption) Writer {
+	w := &defaultWriter{}
+	for _, opt := range opts {
+		opt(&w.WriterConfig)
+	}
+	return w
+}
+
+func escapeRune(writer util.BufWriter, r rune) {
+	if r < 256 {
+		v := util.EscapeHTMLByte(byte(r))
+		if v != nil {
+			_, _ = writer.Write(v)
+			return
+		}
+	}
+	_, _ = writer.WriteRune(util.ToValidRune(r))
+}
+
+func (d *defaultWriter) SecureWrite(writer util.BufWriter, source []byte) {
+	n := 0
+	l := len(source)
+	for i := 0; i < l; i++ {
+		if source[i] == '\u0000' {
+			_, _ = writer.Write(source[i-n : i])
+			n = 0
+			_, _ = writer.Write(replacementCharacter)
+			continue
+		}
+		n++
+	}
+	if n != 0 {
+		_, _ = writer.Write(source[l-n:])
+	}
+}
+
+func (d *defaultWriter) RawWrite(writer util.BufWriter, source []byte) {
+	n := 0
+	l := len(source)
+	for i := 0; i < l; i++ {
+		v := util.EscapeHTMLByte(source[i])
+		if v != nil {
+			_, _ = writer.Write(source[i-n : i])
+			n = 0
+			_, _ = writer.Write(v)
+			continue
+		}
+		n++
+	}
+	if n != 0 {
+		_, _ = writer.Write(source[l-n:])
+	}
+}
+
+func (d *defaultWriter) Write(writer util.BufWriter, source []byte) {
+	escaped := false
+	var ok bool
+	limit := len(source)
+	n := 0
+	for i := 0; i < limit; i++ {
+		c := source[i]
+		if escaped {
+			if util.IsPunct(c) {
+				d.RawWrite(writer, source[n:i-1])
+				n = i
+				escaped = false
+				continue
+			}
+			if d.EscapedSpace && c == ' ' {
+				d.RawWrite(writer, source[n:i-1])
+				n = i + 1
+				escaped = false
+				continue
+			}
+		}
+		if c == '\x00' {
+			d.RawWrite(writer, source[n:i])
+			d.RawWrite(writer, replacementCharacter)
+			n = i + 1
+			escaped = false
+			continue
+		}
+		if c == '&' {
+			pos := i
+			next := i + 1
+			if next < limit && source[next] == '#' {
+				nnext := next + 1
+				if nnext < limit {
+					nc := source[nnext]
+					// code point like #x22;
+					if nnext < limit && nc == 'x' || nc == 'X' {
+						start := nnext + 1
+						i, ok = util.ReadWhile(source, [2]int{start, limit}, util.IsHexDecimal)
+						if ok && i < limit && source[i] == ';' && i-start < 7 {
+							v, _ := strconv.ParseUint(util.BytesToReadOnlyString(source[start:i]), 16, 32)
+							d.RawWrite(writer, source[n:pos])
+							n = i + 1
+							escapeRune(writer, rune(v))
+							continue
+						}
+						// code point like #1234;
+					} else if nc >= '0' && nc <= '9' {
+						start := nnext
+						i, ok = util.ReadWhile(source, [2]int{start, limit}, util.IsNumeric)
+						if ok && i < limit && i-start < 8 && source[i] == ';' {
+							v, _ := strconv.ParseUint(util.BytesToReadOnlyString(source[start:i]), 10, 32)
+							d.RawWrite(writer, source[n:pos])
+							n = i + 1
+							escapeRune(writer, rune(v))
+							continue
+						}
+					}
+				}
+			} else {
+				start := next
+				i, ok = util.ReadWhile(source, [2]int{start, limit}, util.IsAlphaNumeric)
+				// entity reference
+				if ok && i < limit && source[i] == ';' {
+					name := util.BytesToReadOnlyString(source[start:i])
+					entity, ok := util.LookUpHTML5EntityByName(name)
+					if ok {
+						d.RawWrite(writer, source[n:pos])
+						n = i + 1
+						d.RawWrite(writer, entity.Characters)
+						continue
+					}
+				}
+			}
+			i = next - 1
+		}
+		if c == '\\' {
+			escaped = true
+			continue
+		}
+		escaped = false
+	}
+	d.RawWrite(writer, source[n:])
+}
+
+// DefaultWriter is a default instance of the Writer.
+var DefaultWriter = NewWriter()
+
+var bDataImage = []byte("data:image/")
+var bPng = []byte("png;")
+var bGif = []byte("gif;")
+var bJpeg = []byte("jpeg;")
+var bWebp = []byte("webp;")
+var bSvg = []byte("svg+xml;")
+var bJs = []byte("javascript:")
+var bVb = []byte("vbscript:")
+var bFile = []byte("file:")
+var bData = []byte("data:")
+
+// IsDangerousURL returns true if the given url seems a potentially dangerous url,
+// otherwise false.
+func IsDangerousURL(url []byte) bool {
+	if bytes.HasPrefix(url, bDataImage) && len(url) >= 11 {
+		v := url[11:]
+		if bytes.HasPrefix(v, bPng) || bytes.HasPrefix(v, bGif) ||
+			bytes.HasPrefix(v, bJpeg) || bytes.HasPrefix(v, bWebp) ||
+			bytes.HasPrefix(v, bSvg) {
+			return false
+		}
+		return true
+	}
+	return bytes.HasPrefix(url, bJs) || bytes.HasPrefix(url, bVb) ||
+		bytes.HasPrefix(url, bFile) || bytes.HasPrefix(url, bData)
+}
+
+func nodeToHTMLText(n ast.Node, source []byte) []byte {
+	var buf bytes.Buffer
+	for c := n.FirstChild(); c != nil; c = c.NextSibling() {
+		if s, ok := c.(*ast.String); ok && s.IsCode() {
+			buf.Write(s.Text(source))
+		} else if !c.HasChildren() {
+			buf.Write(util.EscapeHTML(c.Text(source)))
+		} else {
+			buf.Write(nodeToHTMLText(c, source))
+		}
+	}
+	return buf.Bytes()
+}