The parent fit plugin extends the data-sizes="auto" feature to also calculate the right sizes for object-fit: contain|cover image elements as also height ( and width) constrained image elements in general.
For this to work properly the physical aspect-ratio of the image candidates need to be calculable. To do so at least one of the image candidates inside the srcset attribute also need to include a h (height) descriptor for at least one image candidate in each srcset.
Simply include this plugin, combine your width descriptors with height descriptors and use object-fit.
<img data-srcset="http://lorempixel.com/400/800/people/6/ 400w 800h,
http://lorempixel.com/300/600/people/6/ 300w,
http://lorempixel.com/200/400/people/6/ 200w"
data-sizes="auto"
class="lazyload"
style="width: 400px; height: 400px; object-fit: contain;" />
<img data-srcset="http://lorempixel.com/800/400/people/9/ 800w 400h,
http://lorempixel.com/600/300/people/9/ 600w,
http://lorempixel.com/400/200/people/9/ 400w"
data-sizes="auto"
class="lazyload"
style="width: 400px; height: 400px; object-fit: cover;" />Due to the fact, that object-fit isn't supported in IE11. This plugin also supports calculating height and width constrained images based on the parent element.
To do so include this plugin, combine your width descriptors with height descriptors and add the attribute data-parent-fit with either "contain" or "cover" as the keyword.
<div style="width: 400px; height: 400px; display: flex; align-items: center; justify-content: center;">
<img data-srcset="http://lorempixel.com/400/800/people/6/ 400w 800h,
http://lorempixel.com/300/600/people/6/ 300w,
http://lorempixel.com/200/400/people/6/ 200w"
data-sizes="auto"
class="lazyload"
data-parent-fit="contain"
style="max-width: 100%; max-height: 100%;" />
</div>In case the width keyword is used, lazySizes simply takes the width of the parent container instead of the img element itself. In this case a h descriptor isn't necessary.
Normally the next closest parent that is not a picture element is used as the parent (i.e.: :not(picture)). This can be changed using the data-parent-container option. It takes any simple selector. If you want to use the viewport as the parent simply add html.
As a special keyword the value self can be used to signalize, that image itself should be taken.
These option can also be set via CSS by abusing the font-family property.
The data-parent-fit option is called here parent-fit and data-parent-container is called parent-container:
img.my-image {
font-family: parent-container: html; parent-fit: contain;
}Note: This plugin should be also added, if you use the bgset plugin in combination with data-sizes="auto" and background-size: cover|contain and it is also the base of the object-fit polyfill plugin.