The Old NetISSUE #6 of 60

all tags lowercase · the case rule

Pixel PeteVSQuirks Mode

Pixel Pete saysIn XHTML, all tag and attribute names must be lowercase — XML is case-sensitive.

HTML treats <DIV> and <div> as the same; XML (and thus XHTML) does not. Every element and attribute name must be lowercase, or a strict parser fails. This is why lowercase markup became the universal convention — it's required in XHTML and good style everywhere.

Power-ups you unlock

XML is case-sensitive; XHTML inherits that
All tags and attributes lowercase
Attribute values can keep their own case
Lowercase is now universal good style

Quirks Mode attacks — common mistakes

Uppercase or MixedCase tag names
Uppercase attribute names like CLASS
Assuming HTML’s case-insensitivity carries over

Boss battleLowercase every tag and attribute name in an uppercase snippet.

Example code

<!doctype html><html><head><meta charset="utf-8"></head>
<body style="background:#06040d;color:#e6e0ff;font-family:monospace;padding:20px"><pre>invalid: &lt;DIV CLASS="X"&gt;
valid:   &lt;div class="X"&gt;</pre></body></html>

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