SEO Tools
Heading outline diff.
Paste the old and new HTML of a page. The tool extracts each H1-H6 outline, aligns them with a longest-common-subsequence diff, and shows what was added, removed, moved or re-levelled.
Inputs
Result
Paste old and new HTML, then press Compare.
How the heading outline diff works
Each input is parsed for native H1 through H6 elements and elements with role="heading" plus aria-level, in source order. The two lists of (level, text) pairs are aligned with a longest-common-subsequence diff, the same idea used by text diff tools. Text is compared after collapsing whitespace and ignoring letter case.
What each label means
- Added / Removed: a heading that exists on only one side.
- Moved: the same level and text appears on both sides but in a different position in the outline.
- Level changed: the same text appears on both sides at a different level, for example H3 became H2.
- H1 count: flagged when the number of H1 headings differs between old and new.
- Skipped levels: flagged in either outline when a heading is more than one level deeper than the one before it, such as H2 followed by H4.
The diff is a structural comparison of heading text and level only. It does not judge whether a change helps or hurts search performance, and duplicate headings are paired in source order.
Sources
- MDN: H1-H6 HTML section heading elements
- MDN: ARIA heading role and aria-level
- Longest common subsequence problem (reference for the diff method)
MDN heading-element and ARIA guidance reviewed September 30, 2026.
Heading outline diff FAQ
Why compare heading outlines before and after a redesign?
Templates and CMS migrations often reorder, rename or re-level headings without anyone noticing. A diff makes those structural changes visible so you can confirm they were intended.
What is the difference between moved and level changed?
Moved means the heading kept its level and text but sits at a different place in the outline. Level changed means the text is the same but the level (H2, H3 and so on) is different.
Does a changed H1 count hurt rankings?
The tool only reports that the count differs. A single descriptive H1 is the clearest pattern, but this tool makes no claim about ranking effects.
Can it fetch two URLs?
No. Browsers block reliable cross-origin fetching, so paste the HTML source of each version.
Is pasted HTML uploaded?
No. Parsing and comparison run locally in your browser, and pasted markup stays inert so embedded resource URLs are not loaded. The page itself still uses the site's analytics and advertising services.