diff --git a/packages/lint/src/rules/core.test.ts b/packages/lint/src/rules/core.test.ts
index 97dac77132..fb0c2ec5e3 100644
--- a/packages/lint/src/rules/core.test.ts
+++ b/packages/lint/src/rules/core.test.ts
@@ -962,6 +962,30 @@ describe("core rules", () => {
       ).toBeUndefined();
     });
 
+    it.each([
+      ["adjacent", `<span>A</span><span>B</span>`],
+      ["spaces", `<span>A</span   ><span>B</span>`],
+      ["newline", `<span>A</span\n    ><span>B</span>`],
+    ])(
+      "does not flag valid sibling spans when the closing tag uses %s whitespace",
+      async (_label, body) => {
+        const result = await lintHyperframeHtml(compositionWithBodyPrefix(body));
+        expect(
+          result.findings.find((f) => f.code === "unclosed_tag_swallowed_element"),
+        ).toBeUndefined();
+      },
+    );
+
+    it.each([`<span class="first" <span>B</span>`, `<span data-label=first <strong>B</strong>`])(
+      "still flags a malformed span start tag that swallows its next element",
+      async (body) => {
+        const result = await lintHyperframeHtml(compositionWithBodyPrefix(body));
+        const finding = result.findings.find((f) => f.code === "unclosed_tag_swallowed_element");
+        expect(finding?.severity).toBe("error");
+        expect(finding?.snippet).toContain("<span");
+      },
+    );
+
     it("does not flag a legitimate attribute value containing a raw <", async () => {
       const html = compositionWithBodyPrefix(`<div data-expr="x < y">hi</div>`);
       const result = await lintHyperframeHtml(html);
diff --git a/packages/lint/src/utils.test.ts b/packages/lint/src/utils.test.ts
index 41663ff9d8..b6d201ec6c 100644
--- a/packages/lint/src/utils.test.ts
+++ b/packages/lint/src/utils.test.ts
@@ -1,9 +1,81 @@
 import { describe, it, expect } from "vitest";
-import { stripCssComments, stripJsComments, stripJsStringLiterals } from "./utils.js";
+import {
+  parseHtmlStructure,
+  stripCssComments,
+  stripJsComments,
+  stripJsStringLiterals,
+} from "./utils.js";
 
 const scan = (src: string) => stripJsStringLiterals(stripJsComments(src));
 const findsRaf = (src: string) => /requestAnimationFrame\s*\(/.test(scan(src));
 
+describe("parseHtmlStructure source ranges", () => {
+  it("does not include ignored markup inside a preceding malformed closing tag", () => {
+    const source = "<p>x</p ignored<noise><span>y</span>";
+    const tags = parseHtmlStructure(source).tags;
+    expect(tags[1]).toMatchObject({ name: "span", raw: "<span>", attrs: "", index: 22 });
+  });
+
+  it("keeps a less-than inside a malformed tag name in the original source range", () => {
+    const tags = parseHtmlStructure("<div<foo>body</div<foo>").tags;
+    expect(tags.map(({ name, raw, attrs, index }) => ({ name, raw, attrs, index }))).toEqual([
+      { name: "div<foo", raw: "<div<foo>", attrs: "", index: 0 },
+    ]);
+  });
+
+  it("uses the original name boundary when Unicode lowercasing changes its length", () => {
+    const source = '<Aİİİ data-check="<" data-flag=x>body</Aİİİ>';
+    const tags = parseHtmlStructure(source).tags;
+    expect(tags.map(({ name, raw, attrs, index }) => ({ name, raw, attrs, index }))).toEqual([
+      {
+        name: "ai̇i̇i̇",
+        raw: '<Aİİİ data-check="<" data-flag=x>',
+        attrs: ' data-check="<" data-flag=x',
+        index: 0,
+      },
+    ]);
+  });
+
+  it("keeps malformed names and quoted less-than values after a multiline close", () => {
+    const source = '<p>x</p\n  ><div<foo data-expr="a < b">y</div<foo>';
+    const tags = parseHtmlStructure(source).tags;
+    expect(tags[1]).toMatchObject({
+      name: "div<foo",
+      raw: '<div<foo data-expr="a < b">',
+      attrs: ' data-expr="a < b"',
+      index: 11,
+    });
+  });
+
+  it("does not reuse implied-open origins for the following explicit tag", () => {
+    const source = '</p></br><Aİ data-expr="x < y">text</Aİ>';
+    const tags = parseHtmlStructure(source).tags;
+    expect(tags.map(({ name, raw, attrs, index }) => ({ name, raw, attrs, index }))).toEqual([
+      { name: "p", raw: "</p>", attrs: "p", index: 0 },
+      { name: "br", raw: "</br>", attrs: "r", index: 4 },
+      { name: "ai̇", raw: '<Aİ data-expr="x < y">', attrs: ' data-expr="x < y"', index: 9 },
+    ]);
+  });
+
+  it("does not use apparent tags in comments or raw-text scripts as an opening origin", () => {
+    const source = '<!-- <fake> --><script>const text = "<fake>";</script\n ><div<foo>x</div<foo>';
+    const tags = parseHtmlStructure(source).tags;
+    expect(tags.map(({ name, raw, attrs }) => ({ name, raw, attrs }))).toEqual([
+      { name: "script", raw: "<script>", attrs: "" },
+      { name: "div<foo", raw: "<div<foo>", attrs: "" },
+    ]);
+  });
+
+  it("starts each open tag at its own < after a multiline closing tag", () => {
+    const source = `<span data-expr="x < y">A</span\n    ><span>B</span>`;
+    const tags = parseHtmlStructure(source).tags;
+    expect(tags.map(({ raw, index }) => ({ raw, index }))).toEqual([
+      { raw: `<span data-expr="x < y">`, index: 0 },
+      { raw: `<span>`, index: source.indexOf("<span>") },
+    ]);
+  });
+});
+
 describe("stripJsStringLiterals", () => {
   it("blanks a call the composition only renders as text", () => {
     expect(findsRaf('const CODE = "requestAnimationFrame(step);";')).toBe(false);
diff --git a/packages/lint/src/utils.ts b/packages/lint/src/utils.ts
index ef24a3a09f..abb15af585 100644
--- a/packages/lint/src/utils.ts
+++ b/packages/lint/src/utils.ts
@@ -70,12 +70,37 @@ export function parseHtmlStructure(source: string): {
     contentStart: number;
     index: number;
   }> = [];
+  let explicitOpenTag: { index: number; nameEnd: number } | null = null;
   const parser: Parser = new Parser(
     {
-      onopentag(name) {
-        const index = parser.startIndex;
+      onopentagname(name) {
+        // startIndex can still point into the preceding close. Bound this scan by
+        // HTML name delimiters, not '<' (which can occur in a malformed name).
+        // Keep the raw name end too: Unicode lowercasing can change UTF-16 length.
+        let tokenStart = parser.endIndex - 1;
+        while (
+          tokenStart >= parser.startIndex &&
+          !/[\t\n\f\r />]/.test(source.charAt(tokenStart))
+        ) {
+          tokenStart -= 1;
+        }
+        const index = source.indexOf("<", tokenStart + 1);
+        explicitOpenTag =
+          index >= 0 &&
+          index < parser.endIndex &&
+          source.slice(index + 1, parser.endIndex).toLowerCase() === name
+            ? { index, nameEnd: parser.endIndex }
+            : null;
+      },
+      onopentag(name, _attrs, isImplied) {
+        const origin = !isImplied ? explicitOpenTag : null;
+        const index = origin?.index ?? parser.startIndex;
+        explicitOpenTag = null;
         const raw = source.slice(index, parser.endIndex + 1);
-        const attrs = raw.slice(name.length + 1, -1).replace(/\s*\/$/, "");
+        const rawAttrs = origin
+          ? source.slice(origin.nameEnd, parser.endIndex)
+          : raw.slice(name.length + 1, -1);
+        const attrs = rawAttrs.replace(/\s*\/$/, "");
         const tag = { raw, name, attrs, index };
         tags.push(tag);
         const sameNameStack = openTagsByName.get(name) ?? [];