Closing (right-side) equal signs are optional.  If both left side and right side equal signs exist, the left side takes precedence. Whitespace **is** allowed before the left-side equal signs.  Only whitespace characters are permitted after the closing equal signs.  No linebreaks are allowed within headings.  No markup within a heading will be parsed.  Also see [[headings reasoning]] for creole's decisions.

Creole:
{{{
= Level 1 (largest) =
== Level 2 ==
=== Level 3 ===
==== Level 4 ====
===== Level 5 =====
====== Level 6 ======
=== Also level 3
=== Also level 3 =
=== Also level 3 ==
=== **not** //parsed// ===
}}}

Recommended XHTML:
{{{
<h1>Level 1 (largest)</h1>
<h2>Level 2</h2>
<h3>Level 3</h3>
<h4>Level 4</h4>
<h5>Level 5</h5>
<h6>Level 6</h6>
<h3>Also level 3</h3>
<h3>Also level 3</h3>
<h3>Also level 3</h3>
<h3>**not** //parsed//</h3>
}}}

[WikiMatrix headings syntax comparison|http://www.wikimatrix.org/syntax.php?i=115]

Unacceptable:
{{{
== Heading with [[link]]
== Heading with **bold** or //italics//
== Heading with {{Image:image|image}}
== Heading with <<6>> placeholder
}}}
----

Also see [All Markup]