HTML Guides for computed styles

This is an internal error from the W3C validator’s CSS checking engine (the Jigsaw CSS validator). The message Cannot invoke "org.w3c.css.values.CssValue.getType()" because "Y" is null is a Java NullPointerException surfacing from the validator’s source code. In practical terms, it means the validator tried to determine the type of a CSS value, but that value didn’t exist or couldn’t be computed.

There are several common causes for this error:

• Malformed or incomplete CSS values — A property is missing part of its value, such as a shorthand with too few components, or a function with missing arguments.
• Unsupported CSS features — Newer CSS features or non-standard syntax that the validator’s CSS engine doesn’t fully support can trigger internal failures rather than clean error messages.
• Invalid value combinations — Using values together in a way that doesn’t match any known CSS grammar rule can cause the parser to fail unexpectedly.
• Syntax errors in custom properties or functions — Mistakes inside calc(), var(), or other CSS functions can sometimes cause this kind of crash in the validator.

While this error technically originates from the validator itself (and could be considered a validator bug), it almost always points to CSS that is non-standard, malformed, or pushing the boundaries of what the validator can parse. Fixing the underlying CSS issue resolves the error.

How to Fix It

1. Locate the CSS property referenced in the error — The "X" in the error message identifies the CSS property or value that caused the failure.
2. Check for syntax errors — Look for missing values, unclosed parentheses, stray commas, or incomplete shorthand declarations.
3. Simplify complex expressions — If you’re using calc(), nested functions, or complex shorthand values, try breaking them into simpler, longhand declarations.
4. Check for unsupported features — Some modern CSS (e.g., certain uses of color-mix(), newer color syntaxes, or container query units) may not yet be supported by the validator. Consider whether validation of that specific rule is critical.
5. Validate CSS separately — Paste your CSS into the W3C CSS Validator directly to isolate the problematic rule.

Examples

Malformed shorthand value

A missing value in a shorthand property can trigger this internal error:

<div style="border: 1px solid;">Content</div>

The border shorthand here has a trailing space after solid with no color specified. While browsers handle this gracefully by using a default, the validator may fail to process the incomplete value. Fix it by providing all intended values explicitly:

<div style="border: 1px solid black;">Content</div>

Incomplete function syntax

Missing arguments inside CSS functions are another common trigger:

<style>
  .box {
    width: calc(100% - );
    background-color: rgb(255, 128);
  }
</style>

The calc() expression is missing its second operand, and rgb() is missing its third argument. Fix both by providing complete values:

<style>
  .box {
    width: calc(100% - 20px);
    background-color: rgb(255, 128, 0);
  }
</style>

Complex expressions the validator cannot parse

Sometimes valid, modern CSS triggers this error because the validator’s engine doesn’t fully support it:

<style>
  .card {
    background: color-mix(in srgb, #3498db 70%, transparent);
  }
</style>

If this triggers the internal error, you can provide a fallback or simplify:

<style>
  .card {
    background: rgba(52, 152, 219, 0.7);
  }
</style>

Accidental double values

A typo that results in two values where one is expected can also cause this:

<style>
  .text {
    font-size: 16px 14px;
  }
</style>

Fix by providing only the single expected value:

<style>
  .text {
    font-size: 16px;
  }
</style>

If you believe your CSS is correct and the validator is producing this error incorrectly, it may be a genuine bug in the validator. You can report it at the W3C CSS Validator’s GitHub repository. In the meantime, check that your CSS works correctly across target browsers and consider adding a /* validated */ comment near the line so you can track intentional exceptions.