CSS: “min-width”: X is not a “min-width” value.

The CSS min-width property has a quoted string as its value, such as "" or "200px", and strings are not valid for this property.

min-width accepts a length (200px, 12rem), a percentage (50%), 0, a calc() expression, the keywords auto, min-content, max-content and fit-content, and CSS-wide keywords like inherit. Anything wrapped in quotes is a CSS string, so min-width: "200px" is rejected even though min-width: 200px is fine. The validator repeats the string in the message: “""” means the declaration was min-width: "", an empty string.

An empty string usually comes from a template or page builder that wraps the value in quotes and prints nothing when the setting is blank. Quoted lengths often come from code copied out of JavaScript or JSON, where values are written as strings (minWidth: "200px").

To fix it, remove the quotes. If there is no value to set, remove the whole declaration; the default min-width is auto. Other bad values produce different messages: a number without a unit (min-width: 200) reports that only 0 can be unitless, and an unknown keyword (min-width: none) reports that values are not recognized.

Invalid example

<style>
  .sidebar {
    min-width: "";
  }

  .card {
    min-width: "200px";
  }
</style>

Valid example

<style>
  .card {
    min-width: 200px;
  }
</style>

The empty declaration on .sidebar is gone, and the .card value has no quotes.

Learn more:

MDN Reference

CSS min-width property - MDN Web Docs

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MDN Reference

CSS length data type - MDN Web Docs

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