Standard HTML

• picture - <picture> element surrounding a <source> tag for each required srcset, and a fallback <img>:

    <picture> 
      <source srcset="..." sizes="..." media="..." type="...">
      <source srcset="..." sizes="..." media="..." type="...">
      (...)
      <img src="...">
    </picture
  

• img - a single <img> tag with a srcset entry:

  <img src="..." srcset="..." sizes="..." alt="..." width="..." height="...">
  

• auto - Supply an img tag when you have only one srcset, otherwise supply a picture tag.