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HTML

The second - alt - script never runs because the page waits for the slow script to finish loading before any more script is executed.

<script type="text/javascript" src="http://www.meouse.com/slowjs.js.php"></script>
<script type="text/javascript">
if( !window.marker ) {
    document.write( 'loading alternative' );
    document.write(
        '<script type="text\/javascript" src="http://www.meouse.com/altjs.js"><\/script>'
    );
}
</script>

<br/><br/>
Contents of slowjs.js.php:<br/>
<pre>
&lt;?php
header( 'Content-Type: text/javascript' );
sleep( 3 );
$js = file_get_contents( 'slowjs.js' );
echo $js;
</pre>

<br/><br/>
Contents of slowjs.js:<br/>
<pre>
if( window.marker ) {
    alert( 'already loaded' );
} else {
    window.marker = true;
    alert( 'first load: slow' );
}
</pre>

<br/><br/>
Contents of altjs.js:<br/>
<pre>
if( window.marker ) {
    alert( 'already loaded' );
} else {
    window.marker = true;
    alert( 'first load: alt' );
}
</pre>