Width Based Srcsets
A width based srcset looks like this:
srcset="myimage-800.jpg 800w, myimage-1200.jpg 1200w, myimage-2000.jpg 2000w"
You should use it when the size of an image depends on the size of the screen used to show it, which generally means anything bigger than about 300 pixels. It's the default; to use it specify a
widths setting (or don't, for the default set), and optionally the
A word on sizes
sizes attribute is both important, and impossible to offer good defaults for. Web browsers parse web pages line-by-line. When they run into an external asset (such as an image) they must download, they start that process immediately without waiting to draw the page. This means that at the point in time when the browser must decide which image to download, it has no clue how large that image will be on the page. The sizes attribute is how we tell it.
It doesn't have to be pixel-perfect, just close enough for the browser to make a good choice. You can't use % (percentage width of the parent container) for the same reason we have to do this at all. If you do not provide it, the web browser will assume the image is 100vw (100% the width of the viewport.)
How to create a sizes attribute
First, Load the page and image as they will appear in the final site. (Basically write the rest of the preset.)
Next, using either dev tools or by manipulating the browser window itself, determine how large the image will be for all reasonable screen sizes. Organize this information into CSS measurements (using
em, or a calculation based on those units) associated with your named CSS media queries, and enter them into the relevant preset. Order matters; enter these from most to least restrictive. The browser will ignore everything after the first media query it finds that is true.
Example: on my particular site, for screens 900px or smaller, inline images are the width of the viewport minus 9px of padding on either side. For screens 901px or larger, they are a constant 862px wide. The relevant lines in my config file look like this:
media_queries: full_width: 'min-width: 901px' # (...) presets: default: # (...) sizes: full_width: 862px size: calc(100vw - 18px)
widths: [integer, integer, (...)]
widths: [600, 800, 1200]
[400, 600, 800, 1000]
Array of image widths to generate, in pixels.
media_widths: (media_query name): [integer, integer, (...)]
media_widths: mobile: [400, 600, 800]
If you are using art direction, there is no sense in generating desktop-size files for your mobile image. Similarly, there's no sense in generating 300px wide versions of your ultrawide crop. You can specify sets of widths to associate with given media queries.
sizes: (media preset): (CSS dimension) (...)
sizes: mobile: 80vw tablet: 60vw desktop: 900px
Conditional sizes, used to construct the
sizes= HTML attribute telling the browser how wide your image will be (on the screen) when a given media query is true. CSS dimensions can be given in
vw. Provide these in order of most restrictive to least restrictive. The browser will choose the first one with an applicable media query.
size: (CSS Dimension)
sizes setting, to be supplied either alone or after all conditional sizes.