The document is not mappable to XML 1.0 due to two consecutive hyphens in a comment.

The HTML specification restricts what can appear inside a comment. Specifically, a comment must not contain the string -- (two consecutive hyphens) except as part of the opening and closing delimiters. This rule originates from SGML and XML 1.0, where -- is treated as a comment delimiter, and having extra occurrences inside the comment body creates ambiguity about where the comment starts and ends.

While most modern browsers are lenient and will handle comments with double hyphens without issues, the markup is technically invalid. This matters for several reasons:

• XML compatibility: If your HTML is served as XHTML or processed by XML parsers, double hyphens inside comments will cause parsing errors and potentially break the entire document.
• Standards compliance: The HTML living standard explicitly states that comments must not contain --, so validators flag this as an error.
• Predictable parsing: Different parsers may interpret malformed comments differently, leading to inconsistent behavior across tools, crawlers, and assistive technologies.

To fix the issue, look inside your comment text and replace any occurrence of -- with something else, such as a single hyphen, an equals sign, or a different separator.

Examples

Incorrect: double hyphens inside the comment body

<!-- This is a separator ---------- end of section -->

The string of hyphens inside the comment contains multiple consecutive -- pairs, making the comment invalid.

<!-- Do not use -- as a separator -->

Here, -- appears literally in the comment text, which violates the rule.

<!-- TODO -- fix this later -- maybe next sprint -->

Multiple -- sequences appear as casual text separators, each one triggering the validation error.

Correct: avoiding double hyphens

Replace double hyphens with a different character or pattern:

<!-- This is a separator ========== end of section -->

<!-- Do not use dashes as a separator -->

<!-- TODO: fix this later, maybe next sprint -->

Correct: standard comment syntax

Simple comments that don’t contain -- in their body are perfectly valid:

<!-- This is a valid comment -->


<!--
  Multi-line comments are fine too,
  as long as they don't contain double hyphens.
-->

Common pitfall: decorative comment borders

Developers sometimes use hyphens to create visual separators in their source code:

<!-- -------------------------------- -->

<!-- Navigation Section               -->

<!-- -------------------------------- -->

Replace these with a different character:

<!-- ================================ -->

<!-- Navigation Section               -->

<!-- ================================ -->

Or simply remove the decorative lines:

<!-- Navigation Section -->

Find issues like this automatically

Rocket Validator scans thousands of pages in seconds, detecting HTML issues across your entire site.

Start Free Trial See Pricing