You can also edit your local copy of the source, and see a preview of the rendered HTML.
As an app, you can also edit the URL, and retrieve the source of any web URL.
To enable the Safari extension on iOS 8, launch Safari, tap the action button on the toolbar, scroll right to the end, then tap More. Make sure ShowSource is enabled, then position it wherever you want in the Safari actions list, and tap Done.
Icons were provided by icon8.com.