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ValidateHTML

Unquoted Attribute Values in XML

In XML, every attribute value must be enclosed in either double quotes or single quotes. Unlike HTML, which sometimes allows unquoted attribute values for simple strings, XML enforces quoting strictly. An unquoted attribute value makes the document not well-formed.

Why It Matters

The XML parser stops at the unquoted attribute and rejects the entire document. This is especially common when developers familiar with HTML write XML and forget that XML has stricter rules for attribute values.

Common Causes

  • Carrying over HTML habits, where simple attribute values can be left unquoted, into XML, which always requires quotes.
  • Hand-writing config or data files quickly and omitting quotes around numeric or single-word values like id=101.
  • Generating XML by string concatenation and forgetting to wrap interpolated values in quotes.

Code Examples

Invalid XML
<?xml version="1.0" encoding="UTF-8"?>
<products>
  <product id=101 category=electronics>
    <name lang=en>Laptop</name>
    <price currency=USD>999</price>
  </product>
</products>
Valid XML
<?xml version="1.0" encoding="UTF-8"?>
<products>
  <product id="101" category="electronics">
    <name lang="en">Laptop</name>
    <price currency="USD">999</price>
  </product>
</products>

How to Fix

  • 1Wrap every attribute value in double quotes: id="101" instead of id=101.
  • 2You can also use single quotes: id='101'. Both are valid in XML.
  • 3If the attribute value contains double quotes, wrap it in single quotes or use &quot; entity.
  • 4Run your XML through a validator to catch all unquoted attributes at once.

Frequently Asked Questions

Can I use single quotes for XML attributes?
Yes. XML accepts both double quotes (id="101") and single quotes (id='101'). They are equivalent. Use single quotes when the value itself contains double quotes, or escape the inner quote with &quot;.
Do numeric attribute values need quotes too?
Yes. XML has no concept of an unquoted or numeric attribute. Every value, including numbers like 101, must be quoted: id="101". Without quotes the document is not well-formed and the parser rejects it.
What if my attribute value contains a quote character?
Wrap the value in the other quote type, or escape the character. Use &quot; for a double quote and &apos; for a single quote inside the value. The ampersand, less-than, and greater-than characters must also be escaped in attribute values.

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