[Jetpack] Remove all CSS.

plugin.php

<?php

// First, make sure Jetpack doesn't concatenate all its CSS
add_filter( 'jetpack_implode_frontend_css', '__return_false' );

// Then, remove each CSS file, one at a time
// You probably won't need them all, unless you use all the modules, and all the themes! :)
// Some of these are also only loaded on specific admin pages, so it wouldn't affect your readers
function jeherve_remove_all_jp_css() {
        wp_deregister_style( 'AtD_style' );                    // After the Deadline
        wp_deregister_style( 'jetpack_likes' );                // Likes
        wp_deregister_style( 'jetpack_related-posts' );        // Related Posts
        wp_deregister_style( 'jetpack-carousel' );             // Carousel
        wp_deregister_style( 'grunion.css' );                  // Contact form
        wp_deregister_style( 'the-neverending-homepage' );     // Infinite Scroll
        wp_deregister_style( 'infinity-twentyten' );           // Infinite Scroll - Twenty Ten
        wp_deregister_style( 'infinity-twentyeleven' );        // Infinite Scroll - Twenty Eleven
        wp_deregister_style( 'infinity-twentytwelve' );        // Infinite Scroll - Twenty Twelve
        wp_deregister_style( 'infinity-twentythirteen' );      // Infinite Scroll - Twenty Thirteen
        wp_deregister_style( 'infinity-twentyfourteen' );      // Infinite Scroll - Twenty Fourteen
        wp_deregister_style( 'infinity-twentyfifteen' );       // Infinite Scroll - Twenty Fifteen
        wp_deregister_style( 'noticons' );                     // Notes
        wp_deregister_style( 'post-by-email' );                // Post by Email
        wp_deregister_style( 'publicize' );                    // Publicize
        wp_deregister_style( 'sharedaddy' );                   // Sharing
        wp_deregister_style( 'sharing' );                      // Sharing
        wp_deregister_style( 'stats_reports_css' );            // Stats
        wp_deregister_style( 'jetpack-widgets' );              // Widgets
        wp_deregister_style( 'jetpack-slideshow' );            // Slideshows
        wp_deregister_style( 'presentations' );                // Presentation shortcode
        wp_deregister_style( 'jetpack-subscriptions' );        // Subscriptions
        wp_deregister_style( 'tiled-gallery' );                // Tiled Galleries
        wp_deregister_style( 'widget-conditions' );            // Widget Visibility
        wp_deregister_style( 'jetpack_display_posts_widget' ); // Display Posts Widget
        wp_deregister_style( 'gravatar-profile-widget' );      // Gravatar Widget
        wp_deregister_style( 'widget-grid-and-list' );         // Top Posts widget
        wp_deregister_style( 'jetpack-widgets' );              // Widgets
}
add_action( 'wp_enqueue_scripts', 'jeherve_remove_all_jp_css' );

It works but not for all styles, ie:
Related Posts: YES
Sharedaddy: NO

Any idea why?

For Sharedaddy / Sharing, I'd guess it's because they give users a UI control for including the CSS & JS

I'd love to know how to get rid of the Jetpack Image Widget CSS.

Just revisited this issue the other day and came up with this:

wp_deregister_style( 'jetpack_image_widget' ); // Image Widgets
wp_dequeue_style( 'jetpack_image_widget' );

Seems to work on my local install.

Worked a treat – thank you kindly.