markdownlint/doc-build/md003.md

This rule is triggered when different heading styles are used in the same
document:

```markdown
# ATX style H1

## Closed ATX style H2 ##

Setext style H1
===============
```

To fix the issue, use consistent heading styles throughout the document:

```markdown
# ATX style H1

## ATX style H2
```

The `setext_with_atx` and `setext_with_atx_closed` settings allow ATX-style
headings of level 3 or more in documents with setext-style headings (which only
support level 1 and 2 headings):

```markdown
Setext style H1
===============

Setext style H2
---------------

### ATX style H3
```

Note: The configured heading style can be a specific style to require (`atx`,
`atx_closed`, `setext`, `setext_with_atx`, `setext_with_atx_closed`), or may
just require that usage is consistent within the document via `consistent`.

Note: The placement of a horizontal rule directly below a line of text can
trigger this rule by turning that text into a level 2 setext-style heading:

```markdown
A line of text followed by a horizontal rule becomes a heading
---
```

Rationale: Consistent formatting makes it easier to understand a document.