Why heading structure is more than just visual styling
Heading tags exist to describe a document's logical outline, not just to make certain lines of text look bigger. A well-structured page uses exactly one H1 for its main topic, then nests H2s as major sections beneath it, H3s as sub-points within each H2, and so on — the same way a well-organized document or book uses chapter and section headings. Both screen readers and search engines rely on this structure to understand how a page's content is organized, independent of the visual font size a designer chose for each level.
This tool parses your pasted HTML and builds the full heading outline as a simple indented tree, so you can see at a glance whether your page's actual heading structure matches the logical structure you intended when writing it. It also runs two specific, well-defined checks: whether the page has exactly one H1, and whether any heading level was skipped on the way down — for example, jumping straight from an H2 to an H4 without an H3 in between.
Why exactly one H1 matters
The H1 is meant to represent a page's single main topic, functioning as the page's own internal headline distinct from (though often similar to) its title tag. A page with zero H1 tags leaves both readers skimming the page and crawlers parsing it without a clear signal of what the page's main subject actually is, relying entirely on the title tag or overall body text to convey that instead. A page with multiple H1 tags dilutes that same signal in the other direction, since it's no longer clear which of several equally-weighted headlines represents the page's actual primary topic — this is a common byproduct of page builder tools or templates that apply H1 styling to multiple elements, such as both a page header and a separate hero banner component.
Why skipped heading levels are worth fixing
Jumping from an H2 directly to an H4, skipping H3 entirely, doesn't usually cause any visible problem — the page still renders and reads fine to a sighted visitor scanning the layout. The issue is structural: a screen reader user navigating a page by heading level, which is a common and efficient way to skip around a long page using assistive technology, experiences a confusing gap when a sub-level is skipped, since the outline they're building in their head no longer matches the page's stated hierarchy. It also weakens the semantic clarity of the page's structure for anything else that relies on that hierarchy, including some crawlers building an understanding of a page's content structure.
Using the outline view while restructuring content
Beyond catching specific errors, the full indented outline this tool produces is useful on its own as a fast way to review whether a long page's structure actually makes sense — whether sections are grouped logically, whether any section has grown so large it deserves to be split into sub-headings, and whether the heading text itself is descriptive enough to make sense on its own, out of context, the way it would appear in a table of contents or a screen reader's heading list.