posts in rows that extend to browser width

Starting a theme from scratch with just post type content and trying to format posts in horizontal rows that extend to the browser width and jump to the row below when browser size is condensed.

Not sure if this is something thats easiest to implement through the loop or the css.

My loop is currently formatted like this:

<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
<div <?php post_class(); ?> id="post">
<h1><?php the_title(); ?></h1>
<h4>Posted on <?php the_time('F jS, Y') ?></h4>
<p><?php the_content(__('(more...)')); ?></p>
</div> <!-- POST -->
<hr> <?php endwhile; else: ?>

Any help appreciated.