# `MD051` - Link fragments should be valid

Tags: `links`

Aliases: `link-fragments`

Fixable: Some violations can be fixed by tooling

This rule is triggered when a link fragment does not match any of the fragments
that are automatically generated for headings in a document:

```markdown
# Heading Name

[Link](#fragment)
```

To fix this issue, change the link fragment to reference an existing heading's
generated name (see below):

```markdown
# Heading Name

[Link](#heading-name)
```

Link fragments may be handled case-sensitively, so this rule requires fragments
to exactly match the [GitHub heading algorithm][github-heading-algorithm].
Therefore, the following example is reported as a violation:

```markdown
# Heading Name

[Link](#Heading-Name)
```

Alternatively, some platforms allow the syntax `{#named-anchor}` to be used
within a heading to provide a specific name (consisting of only lower-case
letters, numbers, `-`, and `_`):

```markdown
# Heading Name {#custom-name}

[Link](#custom-name)
```

Alternatively, any HTML tag with an `id` attribute or an `a` tag with a `name`
attribute can be used to define a fragment:

```markdown
<a id="bookmark"></a>

[Link](#bookmark)
```

An `a` tag can be useful in scenarios where a heading is not appropriate or for
control over the text of the fragment identifier.

This rule also recognizes the custom fragment syntax used by GitHub to highlight
[specific content in a document][github-linking-to-content].

For example, this link to line 20:

```markdown
[Link](#L20)
```

And this link to content starting within line 19 running into line 21:

```markdown
[Link](#L19C5-L21C11)
```

Rationale: [GitHub section links][github-section-links] are created
automatically for every heading when Markdown content is displayed on GitHub.
This makes it easy to link directly to different sections within a document.
However, section links change if headings are renamed or removed. This rule
helps identify broken section links within a document.

Section links are **not** part of the CommonMark specification. This rule
enforces the [GitHub heading algorithm][github-heading-algorithm] which is:
convert heading to lowercase, remove punctuation, convert spaces to dashes,
append an incrementing integer as needed for uniqueness.

[github-section-links]: https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#section-links
[github-heading-algorithm]: https://github.com/gjtorikian/html-pipeline/blob/f13a1534cb650ba17af400d1acd3a22c28004c09/lib/html/pipeline/toc_filter.rb
[github-linking-to-content]: https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-a-permanent-link-to-a-code-snippet#linking-to-markdown#linking-to-markdown