- Retina image\n\n\n// Responsive image\n//\n// Keep images from scaling beyond the width of their parents.\n\n@mixin img-fluid() {\n  // Part 1: Set a maximum relative to the parent\n  max-width: 100%;\n  // Part 2: Override the height to auto, otherwise images will be stretched\n  // when setting a width and height attribute on the img element.\n  height: auto;\n}\n\n\n// Retina image\n//\n// Short retina mixin for setting background-image and -size.\n\n@mixin img-retina($file-1x, $file-2x, $width-1x,...