swc-project/swc · warning · swc_html_parser::error::Error

EndTagViolatesNestingRules

EndTagViolatesNestingRules

Error message

End tag "{tag_name}" violates nesting rules

What it means

Step 6 of the adoption agency algorithm (crates/swc_html_parser/src/parser/mod.rs:7343): when a formatting end tag is processed and the current node (top of the open elements stack) is not the formatting element itself, the end tag 'violates nesting rules'. The parser reports the error and then performs the full adoption agency run — reconstructing active formatting elements and re-parenting nodes — which is exactly the famous `<b><p>x</b>` transformation.

Source

Thrown at crates/swc_html_parser/src/parser/mod.rs:7343

            if formatting_element_stack_index.is_some()
                && !self
                    .open_elements_stack
                    .has_node_in_scope(&formatting_element.1)
            {
                self.errors.push(Error::new(
                    token_and_info.span,
                    ErrorKind::NoElementToCloseButEndTagSeen(subject),
                ));

                return Ok(());
            }

            let formatting_element_stack_index = formatting_element_stack_index.unwrap();

            // 6.
            if let Some(node) = self.open_elements_stack.items.last() {
                if !is_same_node(node, &formatting_element.1) {
                    self.errors.push(Error::new(
                        token_and_info.span,
                        ErrorKind::EndTagViolatesNestingRules(subject.clone()),
                    ));
                }
            }

            // 7.
            let furthest_block = self
                .open_elements_stack
                .items
                .iter()
                .enumerate()
                .skip(formatting_element_stack_index)
                .find(|&(_, open_element)| self.is_special_element(open_element))
                .map(|(i, h)| (i, h.clone()));

            // 8.
            if furthest_block.is_none() {

View on GitHub (pinned to 5176682b65)

Solutions

1. Close the formatting element before opening the block element: <b>1</b><p>2</p>
2. Move the formatting inside the block: <p><b>2</b></p>
3. Use CSS (font-weight, color) instead of wrapping block content in formatting tags
4. Expect re-parented output from recovery — snapshot-test normalized HTML for such inputs

Example fix

<!-- before -->
<b>1<p>2</b>3
<!-- after -->
<b>1</b><p><b>2</b></p>3

// Reject inline formatting elements containing block-level children
const BLOCK = /^(div|p|ul|ol|li|table|tr|td|th|h[1-6]|section|article|blockquote|form|pre|hr)$/i;
function assertNoInlineAroundBlocks(html: string): void {
  for (const m of html.matchAll(/<(b|i|em|strong|u|s|a|span|font)\b[^>]*>([\s\S]*?)<\/\1>/gi)) {
    if (BLOCK.test((m[2].match(/^\s*<([a-zA-Z0-9]+)/) || [])[1] || '')) {
      throw new Error(`<${m[1]}> wraps a block element`);
    }
  }
}

for err in parser.take_errors() {
    if let ErrorKind::EndTagViolatesNestingRules(name) = &err.kind {
        // adoption agency will re-parent nodes; snapshot-test the output
    }
}

When it happens

Trigger: Parsing `<b>1<p>2</b>3` or `<a href><div>click</a>` — a formatting end tag whose element is in scope but not the current node, forcing the parser to run the re-parenting machinery instead of a simple pop.

Common situations: Classic WYSIWYG output (`<b>` wrapped around block elements); anchor tags opened around divs/li; sanitizers stripping block elements inside inline formatting; HTML emails where styling spans wrap paragraphs.

Related errors

• NoElementToCloseButEndTagSeen
• Start tag "{tag_name}" seen but an element of the same type
• No cell to close
• "select" start tag where end tag expected
• "{tag_name}" start tag with "select" open

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/fcc231bc9cae4d4f. Report an issue: GitHub.