The Old NetISSUE #11 of 60

the document must be well-formed

Pixel PeteVSQuirks Mode

Pixel Pete saysAn XHTML document must be "well-formed" — a single valid XML tree with no syntax errors.

"Well-formed" is XML's baseline: one root element, every tag closed and nested, entities escaped, no stray characters. It's separate from "valid" (matching the DTD). A well-formed-but-invalid doc parses; a non-well-formed one fails entirely. Well-formedness is the non-negotiable floor.

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Well-formed = correct XML syntax/tree
One root, closed, nested, escaped
Separate from "valid" (DTD-conforming)
Non-well-formed = total parse failure

Quirks Mode attacks — common mistakes

Confusing well-formed with valid
Unescaped & or < in text
Multiple root elements

Boss battleDistinguish a well-formed-but-invalid doc from a non-well-formed one.

Example code

<!doctype html><html><head><meta charset="utf-8"></head>
<body style="background:#06040d;color:#e6e0ff;font-family:monospace;padding:20px"><pre>not well-formed: &lt;p&gt;a &amp; b   (raw &amp;)
well-formed:     &lt;p&gt;a &amp;amp; b&lt;/p&gt;</pre></body></html>

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