WordPress has a few default classes used for things like image alignment, galleries and other html output by core features. This is a combination of the styles found in the default WordPress TwentyEleven theme, and some modifications of my own. Include this snippet with your css to save a bunch of time building your responsive WordPress theme.

I’ll continue to update this Gist as needed for my own use. Feel free to fork it.

Responsive WordPress Core Theme Styles

by Fork it on Github
Responsive WordPress Core Theme Styles
responsive_wordpress.css
/**
 * Responsive WordPress Core Theme Styles
 * http://jeffsebring.com/responsive-wordpress-images/
--------------------------------------------------- */

.sticky,
.bypostauthor,
.gallery-caption {
   display: normal;
}

.alignnone {
   margin: 1em 1em 1em 0;
}

.aligncenter,
div.aligncenter {
   display:block;
   margin: .5em auto;
}

.alignright {
   float:right;
   margin: 0 0 1em 1em;
}

.alignleft {
   float:left;
   margin: 0 1em 1em 0;
}

.aligncenter {
   display: block;
   margin: 1em auto;
}

img,
img[class*="align"],
img[class*="wp-image-"] {
   max-width: 100%;
   height: auto;
}

img.wp-smiley {
   border: none;
   margin-bottom: 0;
   margin-top: 0;
   padding: 0;
}

img.alignright {
   margin-left: 1em 0 1em 1em;
}

img.alignleft {
   margin: 1em 1em 1em 0;
}

img.aligncenter{
   display: block;
   margin: 1em auto;
}

img#wpstats {
   display: block;
   margin: 0 auto;
}

img[class*="align"],
img[class*="wp-image-"],
   .gallery .gallery-icon img {
   border: none;
}

.wp-caption {
   margin-bottom: 1em;
   margin-left: 0;
   max-width: 96%;
   text-align: center;
}

.wp-caption img {
   display: block;
   margin: 0 auto;
}

.wp-caption-text {
   position: relative;
   font-size: .8em;
}

.gallery {
   margin: 0 auto;
}

.gallery .gallery-item  {
   margin: 0;
   float: left;
   text-align: center;
   width: 33%;
}

.gallery a img {
   border: none;
}

.gallery-columns-4 .gallery-item {
   width: 25%;
}

.gallery-columns-4 .gallery-item img {
   width: 100%;
   height: auto;
}

.comments ol    {
   padding-left: 0;
}

.comments ol li {
   margin: 0;
}

.comment-author {
   padding: 0;
   text-decoration: none;
}


@media only screen and (max-width: 800px) {

   embed,
   object {
      max-width: 100%;
   }

}

@media only screen and ( max-width: 650px ) {

   .gallery-columns-3 .gallery-item {
      width: 33.33%;
   }

   .gallery-columns-3 .gallery-item img {
      width: 100%;
      height: auto;
   }

}

@media only screen and (max-width: 480px) {

   .gallery-columns-2 .gallery-item {
      width: 50%;
   }

   .gallery-columns-2 .gallery-item img {
      width: 100%;
      height: auto;
   }

}