Designer023

IE 7 and 8 don’t do CSS3 or Media queries so instead of adding the fixes to your theme you can add them via a plugin. This means when they finally die a death you can drop support without having to update your theme. This also covers purchased themes so you don’t need to alter the actual files.

Compatiblizr

Plugin for patching old versions of IE (7 and 8)to work with CSS3 selectors and Media Queries.

Jacob Dunn

phenomblue

The PB Responsive Images plugin automatically reformats all images in the post content into a format similar to the picture tag proposed by the <a href="http://www.w3.org/community/respimg/" rel="nofollow ugc">Responsive Images Community Group on w3.org</a>.
The default configuration provides the necessary image sizes for the Twenty Eleven theme for reference; you’ll have to customize the configuration to fit your theme best. Eace image is reformatted based on standard CSS media queries and SLIR image query pairs, giving a huge amount of flexibility in the variety of query combinations. For example, with a media query of <code>(min-width:500)</code> and a resize query of <code>w700</code>, a version of the image at most 700 pixels wide will display only when the screen is greater than 500 pixels wide. Additionally, the plugin provides shortcodes that allow you to customize the queries used per image, and helper functions that can be used to customize the queries used per post or per layout.
For additional information or help on implementing this plugin, use the “Help” tab on the upper right of the plugin screen, which has detailed explanations on each portion of the plugin. If that doesn’t help, please post your questions to <a href="https://wordpress.org/support/plugin/pb-responsive-images" rel="ugc">the forums</a> on wordpress.org.

PB Responsive Images

Adds support for the proposed responsive image format in post content, and helper functions for theme authors.

Compatiblizr vs PB Responsive Images