CSS: “max-width”: “auto” is not a “max-width” value.

The max-width property sets the maximum width an element can grow to, preventing the computed value of width from exceeding the specified limit. While many CSS sizing properties accept auto as a value (for example, width: auto and margin: auto are perfectly valid), the max-width property does not. This is a common mistake because developers often assume auto is universally accepted across similar properties.

When a browser encounters max-width: auto, it will typically ignore the invalid declaration and fall back to the default value of none. While the page may still render as expected in some browsers, relying on this behavior is unreliable and non-standard. Writing valid CSS ensures consistent rendering across all browsers and makes your stylesheets easier to maintain and debug.

If your intent is to remove a maximum width constraint (effectively making max-width have no effect), use none — this is the default value. If you want the element to size itself based on its content, use max-content, min-content, or fit-content. If you need to reset the property to its initial value, use initial (which resolves to none).

Valid values for max-width

The max-width property accepts the following types of values:

• none — No limit on the element’s width (the default).
• Length values — Such as 500px, 3.5em, 20rem, 80ch.
• Percentage values — Such as 75%, relative to the containing block’s width.
• Keyword values — max-content, min-content, fit-content, or fit-content(<length>).
• Global values — inherit, initial, revert, unset.

Examples

❌ Incorrect: using auto with max-width

<div style="max-width: auto;">
  This container has an invalid max-width value.
</div>

This triggers the validation error because auto is not a valid max-width value.

✅ Fixed: using none to remove the constraint

If you want no maximum width limit (the most likely intent when writing auto), use none:

<div style="max-width: none;">
  This container has no maximum width constraint.
</div>

✅ Fixed: using a specific length or percentage

If you want to cap the element’s width at a specific size:

<div style="max-width: 600px;">
  This container will not grow beyond 600 pixels.
</div>

<div style="max-width: 80%;">
  This container will not exceed 80% of its parent's width.
</div>

✅ Fixed: using intrinsic sizing keywords

If you want the element’s maximum width to be based on its content:

<div style="max-width: max-content;">
  This container's max width is determined by its content.
</div>

Saiba mais:

MDN Reference

MDN: CSS max-width

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