A responsive image polyfill for <picture>, srcset, sizes, and more

Since Picturefill doesn't traverse into Shadow DOM it will not be applied to pictures and images inside Shadow DOM. To fix this two custom elements are introduced:

To use them just add is="cs-picturefill-img" or is="cs-picturefill-picture" attributes to your img or picture element correspondingly - this will make them working even inside Shadow DOM.