<s:Button click="showMessage('<h1>Some message</h1>')" label="Как играть?" />
How to put an html message, in the handler, because I get an error of the parser that says, that inside click handlers there not be <
symbols
You can also call your function in this way.
<s:Button label="Как играть?" >
<s:click >
<![CDATA[
showMessage('<h1>Some message</h1>');
]]>
</s:click>
</s:Button>