Weston Ruter

Read full writeup: <a href="https://make.xwp.co/2016/10/29/previewing-themes-with-adaptive-designs-in-the-customizer/" rel="nofollow ugc">Previewing Themes with Adaptive Designs in the Customizer</a>.
This plugin extends the responsive device preview functionality in the customizer which was added in 4.5 (see <a href="https://core.trac.wordpress.org/ticket/31195" rel="nofollow ugc">#31195</a>). In core when you change the previewed device it merely changes the dimensions of the preview, allowing you to simulate how the theme will appear on tablet or mobile screens. What it does not do, however, is cause the preview to reload with the <code>User-Agent</code> overridden for the current device being previewed. For themes that implement a responsive design this core behavior is just fine since changing the dimensions of the iframe will cause the appropriate media queries to apply. However, if you have an adaptive design for your theme you won’t be able to see any server-side components that it may display when viewing a different device. For example, on mobile devices a theme may want to skip outputting the sidebar altogether to save on bandwidth.
This plugin will ensure that the preview is refreshed when the previewed device is changed in addition to changing the preview window size. It will pass a <code>customize_previewed_device</code> query parameter on the URL being previewed, and this parameter will be either <code>desktop</code>, <code>tablet</code>, or <code>mobile</code>. The plugin will override the <code>$_SERVER['HTTP_USER_AGENT']</code> to be a user agent representative of the supplied device type so that calls to <code>wp_is_mobile()</code>, <code>jetpack_is_mobile()</code>, and <code>Jetpack_User_Agent_Info::is_tablet()</code> will all return the expected values based on the current previewed device.
Development of this plugin is done <a href="https://github.com/xwp/wp-customizer-responsive-device-preview" rel="nofollow ugc">on GitHub</a>. Pull requests welcome. Please see <a href="https://github.com/xwp/wp-customizer-responsive-device-preview/issues" rel="nofollow ugc">issues</a> reported there before going to the <a href="https://wordpress.org/support/plugin/customizer-responsive-device-preview" rel="ugc">plugin forum</a>.

Customizer Responsive Server-Side Components Device Preview

Extend device preview in customizer with previewing server-side components, ensuring the previewed device's user agent is set when the site refre …

Stuart Bates

WP Responsive Images once activated automatically detects mobile devices viewing your website and serves up images of appropriate size 
by dynamically resizing images and caching them for improved performance.

WP Responsive Images

WP Responsive Images detects mobile devices and dynamically serves up smaller image sizes to improve page load times and user experience

Customizer Responsive Server-Side Components Device Preview vs WP Responsive Images