This code shows the page differently. Why?

This code:

Code (text):

 <base href="[var.base_url]/">

 <link href="[var.base_url]/themes/default/css/main.css" rel="stylesheet" type="text/css" media="all" />

And this code:

Code (text):

<base href="[var.base_url]/">

<link href="../themes/default/css/main.css" rel="stylesheet" type="text/css" media="all" />

show the page differently. Why? Aren't they pointing to the same css file?

See original post by chrisj