diff --git a/README.md b/README.md
index 032304d..35733d3 100644
--- a/README.md
+++ b/README.md
@@ -161,7 +161,7 @@ You can overwrite the substitutions by `extensions.WithTypographicSubstitutions`
 markdown := goldmark.New(
 	goldmark.WithExtensions(
 		extension.NewTypographer(
-			extension.WithTypographicSubstitutions(extension.TypographerSubstitutions{
+			extension.WithTypographicSubstitutions(extension.TypographicSubstitutions{
 				extension.LeftSingleQuote:  []byte("‚"),
 				extension.RightSingleQuote: nil, // nil disables a substitution
 			}),
diff --git a/extension/typographer.go b/extension/typographer.go
index 9978086..490fb5a 100644
--- a/extension/typographer.go
+++ b/extension/typographer.go
@@ -62,7 +62,7 @@ func newDefaultSubstitutions() [][]byte {
 // SetOption implements SetOptioner.
 func (b *TypographerConfig) SetOption(name parser.OptionName, value interface{}) {
 	switch name {
-	case TypographicSubstitutions:
+	case optTypographicSubstitutions:
 		b.Substitutions = value.([][]byte)
 	}
 }
@@ -73,19 +73,17 @@ type TypographerOption interface {
 	SetTypographerOption(*TypographerConfig)
 }
 
-// TypographicSubstitutions is an otpion name that specify replacement text for
-// punctuations.
-const TypographicSubstitutions parser.OptionName = "TypographicSubstitutions"
+const optTypographicSubstitutions parser.OptionName = "TypographicSubstitutions"
 
-// TypographerSubstitutions is a list of the substitutions for the Typographer extension.
-type TypographerSubstitutions map[TypographicPunctuation][]byte
+// TypographicSubstitutions is a list of the substitutions for the Typographer extension.
+type TypographicSubstitutions map[TypographicPunctuation][]byte
 
 type withTypographicSubstitutions struct {
 	value [][]byte
 }
 
 func (o *withTypographicSubstitutions) SetParserOption(c *parser.Config) {
-	c.Options[TypographicSubstitutions] = o.value
+	c.Options[optTypographicSubstitutions] = o.value
 }
 
 func (o *withTypographicSubstitutions) SetTypographerOption(p *TypographerConfig) {
diff --git a/parser/atx_heading.go b/parser/atx_heading.go
index abfa788..837a113 100644
--- a/parser/atx_heading.go
+++ b/parser/atx_heading.go
@@ -15,9 +15,9 @@ type HeadingConfig struct {
 // SetOption implements SetOptioner.
 func (b *HeadingConfig) SetOption(name OptionName, value interface{}) {
 	switch name {
-	case AutoHeadingID:
+	case optAutoHeadingID:
 		b.AutoHeadingID = true
-	case Attribute:
+	case optAttribute:
 		b.Attribute = true
 	}
 }
@@ -29,13 +29,13 @@ type HeadingOption interface {
 }
 
 // AutoHeadingID is an option name that enables auto IDs for headings.
-var AutoHeadingID OptionName = "AutoHeadingID"
+const optAutoHeadingID OptionName = "AutoHeadingID"
 
 type withAutoHeadingID struct {
 }
 
 func (o *withAutoHeadingID) SetParserOption(c *Config) {
-	c.Options[AutoHeadingID] = true
+	c.Options[optAutoHeadingID] = true
 }
 
 func (o *withAutoHeadingID) SetHeadingOption(p *HeadingConfig) {
diff --git a/parser/html_block.go b/parser/html_block.go
index 00085d3..831ba7d 100644
--- a/parser/html_block.go
+++ b/parser/html_block.go
@@ -17,25 +17,25 @@ type HTMLConfig struct {
 // SetOption implements SetOptioner.
 func (b *HTMLConfig) SetOption(name OptionName, value interface{}) {
 	switch name {
-	case FilterTags:
+	case optFilterTags:
 		b.FilterTags = value.(map[string]bool)
 	}
 }
 
 // A HTMLOption interface sets options for the raw HTML parsers.
 type HTMLOption interface {
+	Option
 	SetHTMLOption(*HTMLConfig)
 }
 
-// FilterTags is an otpion name that specify forbidden tag names.
-const FilterTags OptionName = "FilterTags"
+const optFilterTags OptionName = "FilterTags"
 
 type withFilterTags struct {
 	value map[string]bool
 }
 
 func (o *withFilterTags) SetParserOption(c *Config) {
-	c.Options[FilterTags] = o.value
+	c.Options[optFilterTags] = o.value
 }
 
 func (o *withFilterTags) SetHTMLOption(p *HTMLConfig) {
@@ -43,10 +43,7 @@ func (o *withFilterTags) SetHTMLOption(p *HTMLConfig) {
 }
 
 // WithFilterTags is a functional otpion that specify forbidden tag names.
-func WithFilterTags(names ...string) interface {
-	Option
-	HTMLOption
-} {
+func WithFilterTags(names ...string) HTMLOption {
 	m := map[string]bool{}
 	for _, name := range names {
 		m[name] = true
diff --git a/parser/parser.go b/parser/parser.go
index ad1b3c5..4a17413 100644
--- a/parser/parser.go
+++ b/parser/parser.go
@@ -382,13 +382,13 @@ type Option interface {
 type OptionName string
 
 // Attribute is an option name that spacify attributes of elements.
-const Attribute OptionName = "Attribute"
+const optAttribute OptionName = "Attribute"
 
 type withAttribute struct {
 }
 
 func (o *withAttribute) SetParserOption(c *Config) {
-	c.Options[Attribute] = true
+	c.Options[optAttribute] = true
 }
 
 // WithAttribute is a functional option that enables custom attributes.
diff --git a/renderer/html/html.go b/renderer/html/html.go
index 25284c0..182def9 100644
--- a/renderer/html/html.go
+++ b/renderer/html/html.go
@@ -31,13 +31,13 @@ func NewConfig() Config {
 // SetOption implements renderer.NodeRenderer.SetOption.
 func (c *Config) SetOption(name renderer.OptionName, value interface{}) {
 	switch name {
-	case HardWraps:
+	case optHardWraps:
 		c.HardWraps = value.(bool)
-	case XHTML:
+	case optXHTML:
 		c.XHTML = value.(bool)
-	case Unsafe:
+	case optUnsafe:
 		c.Unsafe = value.(bool)
-	case TextWriter:
+	case optTextWriter:
 		c.Writer = value.(Writer)
 	}
 }
@@ -48,14 +48,14 @@ type Option interface {
 }
 
 // TextWriter is an option name used in WithWriter.
-const TextWriter renderer.OptionName = "Writer"
+const optTextWriter renderer.OptionName = "Writer"
 
 type withWriter struct {
 	value Writer
 }
 
 func (o *withWriter) SetConfig(c *renderer.Config) {
-	c.Options[TextWriter] = o.value
+	c.Options[optTextWriter] = o.value
 }
 
 func (o *withWriter) SetHTMLOption(c *Config) {
@@ -72,13 +72,13 @@ func WithWriter(writer Writer) interface {
 }
 
 // HardWraps is an option name used in WithHardWraps.
-const HardWraps renderer.OptionName = "HardWraps"
+const optHardWraps renderer.OptionName = "HardWraps"
 
 type withHardWraps struct {
 }
 
 func (o *withHardWraps) SetConfig(c *renderer.Config) {
-	c.Options[HardWraps] = true
+	c.Options[optHardWraps] = true
 }
 
 func (o *withHardWraps) SetHTMLOption(c *Config) {
@@ -95,13 +95,13 @@ func WithHardWraps() interface {
 }
 
 // XHTML is an option name used in WithXHTML.
-const XHTML renderer.OptionName = "XHTML"
+const optXHTML renderer.OptionName = "XHTML"
 
 type withXHTML struct {
 }
 
 func (o *withXHTML) SetConfig(c *renderer.Config) {
-	c.Options[XHTML] = true
+	c.Options[optXHTML] = true
 }
 
 func (o *withXHTML) SetHTMLOption(c *Config) {
@@ -118,13 +118,13 @@ func WithXHTML() interface {
 }
 
 // Unsafe is an option name used in WithUnsafe.
-const Unsafe renderer.OptionName = "Unsafe"
+const optUnsafe renderer.OptionName = "Unsafe"
 
 type withUnsafe struct {
 }
 
 func (o *withUnsafe) SetConfig(c *renderer.Config) {
-	c.Options[Unsafe] = true
+	c.Options[optUnsafe] = true
 }
 
 func (o *withUnsafe) SetHTMLOption(c *Config) {