wilsonzlin · vodik · Oct 28, 2025
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -3,6 +3,7 @@
 ## Pending
 
 - Disable treeshake annotations (e.g., `/*#__PURE__*/`) in minified JavaScript output as they are only useful for bundlers, not inline scripts.
+- Add `preserve_self_closing_on_unknown_tags` option to preserve self-closing syntax on unknown HTML elements.
 
 ## 0.18.1
 

diff --git a/minify-html-common/src/spec/tag/whitespace.rs b/minify-html-common/src/spec/tag/whitespace.rs
@@ -58,7 +58,7 @@ static DEFAULT_SVG: &WhitespaceMinification = &WhitespaceMinification {
   trim: true,
 };
 
-static HTML_TAG_WHITESPACE_MINIFICATION: Lazy<
+pub static HTML_TAG_WHITESPACE_MINIFICATION: Lazy<
   AHashMap<&'static [u8], &'static WhitespaceMinification>,
 > = Lazy::new(|| {
   let mut m = AHashMap::<&'static [u8], &'static WhitespaceMinification>::default();

diff --git a/minify-html/src/cfg/mod.rs b/minify-html/src/cfg/mod.rs
@@ -28,6 +28,8 @@ pub struct Cfg {
   pub preserve_brace_template_syntax: bool,
   /// When `<%` is seen in content, all source code until the subsequent matching closing `%>` gets piped through untouched.
   pub preserve_chevron_percent_template_syntax: bool,
+  /// Preserve self-closing syntax on unknown HTML elements (e.g. `<custom />`).
+  pub preserve_self_closing_on_unknown_tags: bool,
   /// Remove all bangs.
   pub remove_bangs: bool,
   /// Remove all processing instructions.

diff --git a/minify-html/src/lib.rs b/minify-html/src/lib.rs
@@ -42,7 +42,7 @@ pub fn minify(src: &[u8], cfg: &Cfg) -> Vec<u8> {
     treat_brace_as_opaque: cfg.preserve_brace_template_syntax,
     treat_chevron_percent_as_opaque: cfg.preserve_chevron_percent_template_syntax,
   });
-  let parsed = parse_content(&mut code, Namespace::Html, EMPTY_SLICE, EMPTY_SLICE);
+  let parsed = parse_content(cfg, &mut code, Namespace::Html, EMPTY_SLICE, EMPTY_SLICE);
   let mut out = Vec::with_capacity(src.len());
   minify_content(
     cfg,
@@ -58,7 +58,8 @@ pub fn minify(src: &[u8], cfg: &Cfg) -> Vec<u8> {
 
 pub fn canonicalise<T: Write>(out: &mut T, src: &[u8]) -> std::io::Result<()> {
   let mut code = Code::new(src);
-  let parsed = parse_content(&mut code, Namespace::Html, EMPTY_SLICE, EMPTY_SLICE);
+  let cfg = Cfg::new();
+  let parsed = parse_content(&cfg, &mut code, Namespace::Html, EMPTY_SLICE, EMPTY_SLICE);
   for c in parsed.children {
     c14n_serialise_ast(out, &c)?;
   }

diff --git a/minify-html/src/parse/content.rs b/minify-html/src/parse/content.rs
@@ -1,4 +1,5 @@
 use crate::ast::NodeData;
+use crate::cfg::Cfg;
 use crate::entity::decode::decode_entities;
 use crate::parse::bang::parse_bang;
 use crate::parse::comment::parse_comment;
@@ -169,6 +170,7 @@ pub struct ParsedContent {
 
 // Use empty slice for `grandparent` or `parent` if none.
 pub fn parse_content(
+  cfg: &Cfg,
   code: &mut Code,
   ns: Namespace,
   grandparent: &[u8],
@@ -228,7 +230,7 @@ pub fn parse_content(
     };
     match typ {
       Text => break,
-      OpeningTag => nodes.push(parse_element(code, ns, parent)),
+      OpeningTag => nodes.push(parse_element(cfg, code, ns, parent)),
       ClosingTag => {
         closing_tag_omitted = false;
         break;

diff --git a/minify-html/src/parse/element.rs b/minify-html/src/parse/element.rs
@@ -2,6 +2,7 @@ use crate::ast::AttrVal;
 use crate::ast::ElementClosingTag;
 use crate::ast::NodeData;
 use crate::ast::ScriptOrStyleLang;
+use crate::cfg::Cfg;
 use crate::entity::decode::decode_entities;
 use crate::parse::content::parse_content;
 use crate::parse::content::ParsedContent;
@@ -22,6 +23,7 @@ use minify_html_common::gen::codepoints::WHITESPACE_OR_SLASH_OR_EQUALS_OR_RIGHT_
 use minify_html_common::spec::script::JAVASCRIPT_MIME_TYPES;
 use minify_html_common::spec::tag::ns::Namespace;
 use minify_html_common::spec::tag::void::VOID_TAGS;
+use minify_html_common::spec::tag::whitespace::HTML_TAG_WHITESPACE_MINIFICATION;
 use std::fmt::Debug;
 use std::fmt::Formatter;
 use std::str::from_utf8;
@@ -131,7 +133,7 @@ pub fn parse_tag(code: &mut Code) -> ParsedTag {
 }
 
 // `<` must be next. `parent` should be an empty slice if it doesn't exist.
-pub fn parse_element(code: &mut Code, ns: Namespace, parent: &[u8]) -> NodeData {
+pub fn parse_element(cfg: &Cfg, code: &mut Code, ns: Namespace, parent: &[u8]) -> NodeData {
   let ParsedTag {
     name: elem_name,
     attributes,
@@ -146,7 +148,15 @@ pub fn parse_element(code: &mut Code, ns: Namespace, parent: &[u8]) -> NodeData
   };
 
   // Only foreign elements can be self closed.
-  if self_closing && ns != Namespace::Html {
+  // However, if preserve_self_closing_on_unknown_tags is enabled,
+  // preserve self-closing syntax on unknown HTML elements as well.
+  let should_preserve_self_closing = cfg.preserve_self_closing_on_unknown_tags
+    && !VOID_TAGS.contains(elem_name.as_slice())
+    && HTML_TAG_WHITESPACE_MINIFICATION
+      .get(elem_name.as_slice())
+      .is_none();
+
+  if self_closing && (ns != Namespace::Html || should_preserve_self_closing) {
     return NodeData::Element {
       attributes,
       children: Vec::new(),
@@ -183,7 +193,7 @@ pub fn parse_element(code: &mut Code, ns: Namespace, parent: &[u8]) -> NodeData
     (_, b"style") => parse_style_content(code),
     (Namespace::Html, b"textarea") => parse_textarea_content(code),
     (Namespace::Html, b"title") => parse_title_content(code),
-    _ => parse_content(code, ns, parent, &elem_name),
+    _ => parse_content(cfg, code, ns, parent, &elem_name),
   };
 
   if !closing_tag_omitted {

diff --git a/minify-html/src/parse/tests/element.rs b/minify-html/src/parse/tests/element.rs
@@ -1,6 +1,7 @@
 use crate::ast::AttrVal;
 use crate::ast::ElementClosingTag;
 use crate::ast::NodeData;
+use crate::cfg::Cfg;
 use crate::parse::element::parse_element;
 use crate::parse::element::parse_tag;
 use crate::parse::element::ParsedTag;
@@ -52,7 +53,8 @@ fn test_parse_tag() {
 #[test]
 fn test_parse_element() {
   let mut code = Code::new(br#"<a b=\"c\"></a>"#);
-  let elem = parse_element(&mut code, Namespace::Html, EMPTY_SLICE);
+  let cfg = Cfg::new();
+  let elem = parse_element(&cfg, &mut code, Namespace::Html, EMPTY_SLICE);
   assert_eq!(elem, NodeData::Element {
     attributes: {
       let mut map = AHashMap::<Vec<u8>, AttrVal>::default();

diff --git a/minify-html/src/tests/mod.rs b/minify-html/src/tests/mod.rs
@@ -232,3 +232,21 @@ fn test_style_attr_minification() {
   // `style` attributes are removed if fully minified away.
   eval_with_css_min(br#"<div style="  /*  */   "></div>"#, br#"<div></div>"#);
 }
+
+#[test]
+fn test_preserve_self_closing_on_unknown_tags() {
+  let mut cfg = Cfg::new();
+  cfg.preserve_self_closing_on_unknown_tags = true;
+  eval_with_cfg(b"<custom />", b"<custom/>", &cfg);
+  eval_with_cfg(b"<custom class=\"foo\" />", b"<custom class=foo />", &cfg);
+  eval_with_cfg(b"<custom-element />", b"<custom-element/>", &cfg);
+  eval_with_cfg(b"<my-component />", b"<my-component/>", &cfg);
+  eval_with_cfg(b"<div />", b"<div>", &cfg);
+  eval_with_cfg(b"<div id=\"test\" />", b"<div id=test>", &cfg);
+  eval_with_cfg(b"<span />", b"<span>", &cfg);
+  eval_with_cfg(b"<input />", b"<input>", &cfg);
+  eval_with_cfg(b"<br />", b"<br>", &cfg);
+  eval_with_cfg(b"<img />", b"<img>", &cfg);
+  eval_with_cfg(b"<svg><path /></svg>", b"<svg><path/></svg>", &cfg);
+  eval_with_cfg(b"<svg><circle /></svg>", b"<svg><circle/></svg>", &cfg);
+}