.
==Mark==
.
<p><mark>Mark</mark></p>
.


These are not marks, you have to use exactly two "==":
.
x ===foo===

x ==foo===

x ===foo==
.
<p>x ===foo===</p>
<p>x ==foo===</p>
<p>x ===foo==</p>
.

Marks have the same priority as emphases:

.
**==test**==

==**test==**
.
<p><strong>==test</strong>==</p>
<p><mark>**test</mark>**</p>
.

Marks have the same priority as emphases with respect to links:
.
[==link]()==

==[link==]()
.
<p><a href="">==link</a>==</p>
<p>==<a href="">link==</a></p>
.

Marks have the same priority as emphases with respect to backticks:
.
==`code==`

`==code`==
.
<p>==<code>code==</code></p>
<p><code>==code</code>==</p>
.

Nested marks:
.
==foo ==bar== baz==
.
<p><mark>foo <mark>bar</mark> baz</mark></p>
.

.
==f **o ==o b== a** r==
.
<p><mark>f <strong>o <mark>o b</mark> a</strong> r</mark></p>
.

Should not have a whitespace between text and "==":
.
foo == bar == baz
.
<p>foo == bar == baz</p>
.


Newline should be considered a whitespace:

.
==test
==

==
test==

==
test
==
.
<h1>==test</h1>
<p>==
test==</p>
<p>==
test
==</p>
.