Bad value for attribute “action” on element “form”: Percentage ("%") is not followed by two hexadecimal digits.

In URLs, certain characters must be percent-encoded — a process where a character is replaced by % followed by exactly two hexadecimal digits representing its byte value. For example, a space becomes %20, a hash becomes %23, and an ampersand becomes %26. The % character itself is the escape prefix, so when a URL parser encounters %, it expects the next two characters to be valid hexadecimal digits (0–9, A–F, a–f). If they aren’t, the URL is malformed.

This validation error typically arises in a few scenarios:

• A literal % is used in the URL without encoding. For instance, a filename or path segment contains a % sign that wasn’t converted to %25.
• An incomplete or corrupted percent-encoding sequence. Someone may have partially encoded a URL, leaving behind sequences like %G5 or %2 that don’t form valid two-digit hex codes.
• Copy-paste errors. A URL was pasted from a source that stripped characters or introduced stray % symbols.

This matters because browsers may interpret malformed URLs inconsistently. One browser might try to “fix” the URL by encoding the stray %, while another might pass it through as-is, leading to broken form submissions or unexpected server-side behavior. Ensuring valid URLs in the action attribute guarantees predictable behavior across all browsers and complies with both the WHATWG URL Standard and the HTML specification.

How to Fix

1. Locate every % in the URL. Check whether each % is followed by exactly two hexadecimal digits (e.g., %20, %3A, %7E).
2. Encode literal % characters. If a % is meant to appear as a literal character in the URL (not as part of a percent-encoding sequence), replace it with %25.
3. Fix incomplete sequences. If a sequence like %2 or %GZ exists, determine the intended character and encode it correctly, or remove the stray %.
4. Use proper encoding tools. In JavaScript, use encodeURIComponent() or encodeURI() to safely encode URLs. Most server-side languages have equivalent functions (e.g., urlencode() in PHP, urllib.parse.quote() in Python).

Examples

Literal % not encoded

This triggers the error because %d is not a valid percent-encoding sequence (d is only one character, and what follows may not be hex):

<!-- ❌ Bad: bare % not followed by two hex digits -->

<form action="/search?discount=20%off">
  <button type="submit">Search</button>
</form>

Encode the % as %25:

<!-- ✅ Good: literal % encoded as %25 -->

<form action="/search?discount=20%25off">
  <button type="submit">Search</button>
</form>

Incomplete percent-encoding sequence

Here, %2 is missing its second hex digit:

<!-- ❌ Bad: %2 is an incomplete sequence -->

<form action="/path/to%2file.html">
  <button type="submit">Submit</button>
</form>

If the intent was to encode a / character (%2F), complete the sequence:

<!-- ✅ Good: complete percent-encoding for "/" -->

<form action="/path/to%2Ffile.html">
  <button type="submit">Submit</button>
</form>

Invalid hex characters after %

The characters G and Z are not hexadecimal digits:

<!-- ❌ Bad: %GZ is not valid hex -->

<form action="/data%GZprocess">
  <button type="submit">Go</button>
</form>

If %GZ was never intended as encoding, escape the % itself:

<!-- ✅ Good: literal % properly encoded -->

<form action="/data%25GZprocess">
  <button type="submit">Go</button>
</form>

Using JavaScript to encode safely

When building URLs dynamically, use built-in encoding functions to avoid this issue entirely:

const query = "20% off";
const safeURL = "/search?q=" + encodeURIComponent(query);
// Result: "/search?q=20%25%20off"

This ensures every special character — including % — is properly percent-encoded before it’s placed in the action attribute.

Más información:

MDN Reference

HTML form element - action attribute

→

WHATWG Spec

HTML forms

→

WHATWG Spec

URL Living Standard

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