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How our HTML Validator Works

In this article, you will learn how our HTML Validator checks code when submitted to our autograder.

Last updated - January 16, 2026

This feature is currently only available on the Voyage platform and must be enabled by the educator in the section's settings.

How the validator processes your code

When submitting HTML or CSS code to a coding challenge, your code may be passed through our HTML Validator. This program checks if any syntax errors are present in your files and will not let you pass a challenge until they are fixed.

If the HTML Validator catches an error, it will display a red message above the requirements showing what error(s) are present in the code:

d1-3

Our validator uses third-party packages to check HTML and CSS code, which include pre-defined rules that determine what gets flagged. Here is a list of enabled rules as of January 15, 2026:

Rule: Description Status (true/false)

alt-require: The alt attribute of an img element must be present and alt attribute of area[href] and input[type=image] must have a value.

 true

attr-lowercase: All attribute names must be in lowercase.

 true

attr-no-duplication: Elements cannot have duplicate attributes.

 true

attr-unsafe-chars: Attribute values cannot contain unsafe chars.

 false

attr-value-double-quotes: Attribute values must be in double quotes.

 true

doctype-first: <doctype> must be declared first.

 false

head-script-disabled:  The <script> tag cannot be used in <head> tag.

 false

id-unique: The value of id attributes must be unique.

 true

inline-style-disabled: Inline style cannot be used.

 false

space-tab-mixed-disabled: Do not mix tabs and spaces for indentation.

 false

spec-char-escape: Special characters must be escaped.

 false

src-not-empty:The src attribute of an img(script,link) must have a value.

 true

style-disabled: <style> tags cannot be used.

 false

tag-pair: Tag must be paired.

 true

tag-self-close: Empty tags must be self closed.

 true

tagname-lowercase: All HTML element names must be in lowercase.

true

Any syntax issues beyond these rules will not be caught by the HTML Validator.

 

How to toggle on/off the HTML Validator (Educators only)

The HTML Validator setting can be toggled on or off in the Manage Sections page of the Educator Portal. Clicking the pencil iconlabeled Edit Section will open a pop-up window, and the toggle will be displayed when scrolling down.

manage-sections-edit-section

edit-section-pop-up

HTML Validator Tips

  • When the validator catches an error, it will display a red error message above the requirements showing what error(s) are present.
  • The message will also include tips that will help students fix their code. Note that it may not state the exact problem all the time.
  • Remember that coding is specific. Make sure all code has correct spelling, syntax, and spacing.
  • Just because code successfully runs in the terminal and meets all of the challenge requirements, it does not mean tit will pass the validator.
  • Check your code, double-check, and check again! If the submitted code is not passing, look it over for bugs. A bug can be as small as a missing bracket or a misspelled word!

If a student is unable to pass an activity, you can manually mark their challenge as complete in the Student Progress page of the Educator Portal.