Heading levels must nest sequentially and must not skip levels for visual effect

What it is

Making a line of type slightly smaller does not licence skipping a level in the structure. Heading levels nest sequentially: under a level-one sits a level-two, under a two a three. Do not jump from two to four because “this block should look secondary.” Assistive technology walks an outline by level; a skip is read as a missing layer in the structure, not as “smaller type.”

Size, weight and colour are appearance. Level is parent–child in the outline. The two knobs must be turned separately.

Why it happens

A heading outline is a tree. Each heading declares “I am a child of the level above,” not “I am bigger than nearby type.” Skipping a level inserts a node with no father: moving from two to four, a user thinks they missed a three and goes looking for a level that does not exist. Visual design often skips to make rhythm — a subhead styled as a four because a two looks too large — which treats an appearance scale as a structure scale.

The right split: keep a three in the tree, and style that three down to the size you want. The tree stays whole; visual rhythm can still exist. The inverse error, marking a true four as a two, promotes unrelated blocks into siblings and fills the outline with false peer sections.

Studying it

Extract the page’s heading tree (accessibility tree or the reader’s heading list), mark every numeric skip, then compare with the visual comp to see whether the skip exists only for type size. Ask screen-reader users to walk a page from the heading list alone and report where they felt “a layer is missing.”

Independent variables: whether skips are allowed, whether a skip tracks a visual type ladder, whether style is decoupled from level. Dependent variables: outline-gap count, how often users report a level they cannot find, time to locate a section.

Automation can list the heading-number sequence; it cannot tell a genuine missing chapter from appearance abuse. The latter needs the design intent. Sample marketing landing pages — they skip most often for rhythm.

Where it stops holding

An aggregation page that embeds several independent articles, each starting from its own top level, looks like several trees side by side; that is not an intra-article skip. Legal or scholarly sources that already skip numbering should be republished with the original structure plus a note, not rewritten “to be sequential.” A jump from one to three is sometimes genuine absence of a middle layer — then the question is whether the three should be promoted to two, not whether an empty middle heading should be invented. Visual documents (slides, posters) often have only a visual hierarchy and no heading tree; the tree rule applies only once heading elements exist.

Applying it

• Choose level from parent–child content: same depth, same level; the next depth uses only the next number. Do not skip a number to make type smaller.
• When smaller appearance is needed, change the style, not the level.
• If a two is followed immediately by a four, ask whether the missing chapter exists: if yes, add it; if no, promote the four to three.
• How to check: open the reader’s heading list (or the accessibility tree) and read the level numbers top to bottom. For every skip, ask whether a chapter is truly missing or the skip is visual. Convert the latter back to sequential levels.

Related

• Same group: J2.14.2 A page typically has one top-level heading that names the page topic · J2.14.3 Landmark regions (navigation, main, footer) supply structure beyond headings · J2.14.4 Marking non-heading emphasis as headings dilutes their navigation value
• Nearby: J2.09 Semantic Structure · J5.01 Screen Readers · J5.02 Accessibility Tree and Roles
• Search terms: heading hierarchy · skipped heading level · sequential headings