swc-project/swc · warning · Error

Start tag "{tag_name}" seen but an element of the same type

Error message

Start tag "{tag_name}" seen but an element of the same type was already open

What it means

A start tag `a` was found while the list of active formatting elements already contains an `a` element between the end of the list and the last marker (parser/mod.rs:3494-3515). HTML forbids nested anchors, so the parser records `SomethingSeenWhenSomethingOpen("a")`, runs the adoption agency algorithm, and removes the old anchor from the active formatting list and the open elements stack before inserting the new one.

Source

Thrown at crates/swc_html_parser/src/parser/mod.rs:3513

                            let mut node = None;

                            for element in self.active_formatting_elements.items.iter().rev() {
                                match element {
                                    ActiveFormattingElement::Marker => {
                                        break;
                                    }
                                    ActiveFormattingElement::Element(item, _) => {
                                        if is_html_element!(item, "a") {
                                            node = Some(item);

                                            break;
                                        }
                                    }
                                }
                            }

                            if let Some(element) = node {
                                self.errors.push(Error::new(
                                    token_and_info.span,
                                    ErrorKind::SomethingSeenWhenSomethingOpen(tag_name.clone()),
                                ));

                                let remove = element.clone();

                                self.run_the_adoption_agency_algorithm(token_and_info, false)?;
                                self.active_formatting_elements.remove(&remove);
                                self.open_elements_stack.remove(&remove);
                            }
                        }

                        self.reconstruct_active_formatting_elements()?;

                        let element = self.insert_html_element(token_and_info)?;

                        self.active_formatting_elements
                            .push(ActiveFormattingElement::Element(

View on GitHub (pinned to 5176682b65)

Solutions

1. Close the previous anchor before opening the next: `<a>first</a> <a>second</a>`
2. Fix the template loop/generator so each emitted anchor is fully closed
3. Sanitize input with a whitelist-based sanitizer that auto-balances tags before parsing
4. Check `take_errors()` for `SomethingSeenWhenSomethingOpen` and reject/repair nested-link input

Example fix

<!-- before -->
<a href="/a">first <a href="/b">second</a></a>

<!-- after -->
<a href="/a">first</a> <a href="/b">second</a>

// Detect nested anchors before parsing
fn has_nested_anchor(src: &str) -> bool {
    let mut a_open = false;
    for tag in html_tag_tokens(src) {
        match tag {
            Start(ref n) if n == "a" => { if a_open { return true; } a_open = true; }
            End(ref n) if n == "a" => a_open = false,
            _ => {}
        }
    }
    false
}

fn is_nested_anchor_error(e: &Error) -> bool {
    matches!(e.kind, ErrorKind::SomethingSeenWhenSomethingOpen(ref t) if &**t == "a")
}

let doc = parser.parse_document()?;
let nested = parser.take_errors().into_iter().any(is_nested_anchor_error);
if nested {
    // adoption agency already flattened the anchors; decide whether to reject input
    return Err(ContentRejected::NestedAnchors);
}

When it happens

Trigger: Input like `<a href="/1">first <a href="/2">second</a></a>` — the second `<a>` start tag with an unclosed `<a>` still active. Also happens when templating loops concatenate link partials without closing the previous anchor.

Common situations: CMS rich-text editors that allow nested links, string concatenation of link fragments in server templates (`"<a>" + user1 + "<a>" + user2`), and scraped/legacy pages. Browsers silently unwrap the nesting; this error tells you the tree will be reshaped by the adoption agency algorithm.

Related errors

• HTML start tag "{tag_name}" in a foreign namespace context
• End tag "{end_tag_name}" did not match the name of the curre
• UnclosedElements
• Start tag for "table" seen but the previous "table" is still
• failed to parse input as document fragment

AI-assisted analysis of swc-project/swc@5176682b65 (2026-08-17). Data as JSON: /api/errors/2de742aca7e7857a. Report an issue: GitHub.