html: add "GFM quirks" mode.

src/html.rs

@@ -774,8 +774,9 @@ impl<'o> HtmlFormatter<'o> {
             }
             NodeValue::Strong => {
                 let parent_node = node.parent();
-                if parent_node.is_none()
-                    || !matches!(parent_node.unwrap().data.borrow().value, NodeValue::Strong)
+                if !self.options.render.gfm_quirks
+                    || (parent_node.is_none()
+                        || !matches!(parent_node.unwrap().data.borrow().value, NodeValue::Strong))
                 {
                     if entering {
                         self.output.write_all(b"<strong")?;

src/main.rs

@@ -56,8 +56,10 @@ struct Cli {
     #[arg(long)]
     full_info_string: bool,
 
-    /// Enable GitHub-flavored markdown extensions: strikethrough, tagfilter, table, autolink, and tasklist.
-    /// Also enables --github-pre-lang.
+    /// Enable GitHub-flavored markdown extensions: strikethrough, tagfilter,
+    /// table, autolink, and tasklist. Also enables --github-pre-lang, and
+    /// enables GFM-style quirks in output HTML, such as not nesting <strong>
+    /// tags, which otherwise breaks CommonMark compatibility.
     #[arg(long)]
     gfm: bool,
 
@@ -284,6 +286,7 @@ fn main() -> Result<(), Box<dyn Error>> {
         .escaped_char_spans(cli.escaped_char_spans)
         .ignore_setext(cli.ignore_setext)
         .ignore_empty_links(cli.ignore_empty_links)
+        .gfm_quirks(cli.gfm)
         .build()?;
 
     let options = Options {

src/parser/mod.rs

@@ -766,6 +766,22 @@ pub struct RenderOptions {
     /// assert_eq!(markdown_to_html(input, &options), "<p>[]()</p>\n");
     /// ```
     pub ignore_empty_links: bool,
+
+    /// Enables GFM quirks in HTML output which break CommonMark compatibility.
+    ///
+    /// ```rust
+    /// # use comrak::{markdown_to_html, Options};
+    /// let mut options = Options::default();
+    /// let input = "****abcd**** *_foo_*";
+    ///
+    /// assert_eq!(markdown_to_html(input, &options),
+    ///            "<p><strong><strong>abcd</strong></strong> <em><em>foo</em></em></p>\n");
+    ///
+    /// options.render.gfm_quirks = true;
+    /// assert_eq!(markdown_to_html(input, &options),
+    ///            "<p><strong>abcd</strong> <em><em>foo</em></em></p>\n");
+    /// ```
+    pub gfm_quirks: bool,
 }
 
 #[non_exhaustive]