How to Use Web Responsive Images


In the world of web design can give a responsive core, but spec complex you important to reduce the size of the page on the spectrum of the device. In this post, I show the complexity of specifying sensitive images, so you can use this powerful HTML attributes in your site. In Part 2, you will learn how you can build your own reactive image workflow, with a demo code distilling our stack of sensitive images into a single file. In addition, we will explore how we automate reactive scale images to process millions of AWS Lambda Webflow images.

Responsive Images on the web today

The <img> was close to long. Give it a src attribute, and you’re on the right track. The specification adds two new attributes that the browser responds to an image.

The new attributes are sizes and srcset. To put it simply: the size of the browser gives the size of the <img> element will give srcset and gives the browser a list of variant images to choose from. The goal is to toggle the browser differs srcset to download as quickly as possible.

The browser has srcset the attributes and size you offer, combines them with the density of the window width and the screen already know and can start the correct image variant after the html analyzed first is done. For and javascript are css same loaded. Modern browsers picking pre-activated can start downloading the correct version before browsing even on the page. This is a huge performance for end users!

To see in action, see Webflow Image Features and open network inspector to see that the browser loads the correct versions.

Responsive Attributes

How to Use Srcset

Srcset is simply a list of image variants. You can specify a pixel density for each variant in the list as srcset = “http: //variant-1.jpg 2x, http: //variant-2.jpg 1,5x”. However, this format only solves the hardware, which provides better quality images with better screens, and does little for responsive design.

What you really want is to mention the variations in pixel width, so if your site is loaded on a mobile format and 500px is widely displayed or desktop layout at 750px wide, it only downloads the Variant desired for this layout make it out. The width width similar to that-srcset = “http: // variant 1jpg-500w, http: //variant-2.jpg 750w, http: //variant-3.jpg 1000w, http: // variant 4-1500w .jpg “. here the pixel width of the actual image file that represents the corresponding URL points.

Image Code

Do not worries, depending on the pixel width method you get free pixel density; your browser knows what it does. Given the previous example on the layout of the mobile browser would you choose the 500px image on a 1x screen and 1000px wide image to download a 2x screen.

“What about the height of the image? You say. In truth, I say: “The height is irrelevant, Let’s not print, do not let me pose vertical videos Let’s go.”

How to use Sizes

The sizes are much less simple. It has things that are known, but cannot be summarized, and there are reasons for reasons out there. Implemented after extensive debate amongst the standards holders; But even after understanding the reasons and accepting them, quite frustrating to come and the supernatural testimony. Unlike srcset which has shown image variations and never have to change once you have done, measurements can be updated when you add a new feature or adjust your CSS because it sets the size of The displayed image. Sort of.

The size of an intrinsic image

Our goal is to tell the browser the size of the <img> will do before it is generated. The browser trusts the sizes attribute if implicitly it covers the width described as a fact that the actual size of the image variant looks after being downloaded. This is called intrinsic size.

It is not quite true to say that the size attribute defines the size at which a pixel is displayed. What it really does is set the size of the asset that best fits the layout.

Suppose you have a container element that extends hovering and a <img> element that is up to 100% so that it fills the container without having to be larger than the actual width of the file, picture. With just one src attribute, we can predict how a smaller image than the floating state and one equal or greater will be carried over the width of the floating state. With a single src attribute, the browser determines the intrinsic size by measuring the actual image file.

Image Code

When using measurements, however, the browser applies the same behavior, but instead of measuring the variation once downloaded, the intrinsic size determines all the size of the attributes. It is as if the browser creates a new image file in memory dimensions that match what is defined in the size of the attributes but filled in with the details of the actual movement they choose to download the src set. Providing variants that are superior to the intrinsic size will ensure that the data shrinks so that they integrate well, but only small variations that what is needed, the browser will force to extend the data into memory-fill image. This can be very striking, so it is important to get the appropriate size attribute.

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When the browser evaluates the partners’ characteristics?

You can expect in the case above you update the size attribute when you need the container to float would change the intrinsic size. But the browser has already made the image in memory and does not create another with a new size of the asset unless the attribute changes srcset. The downloaded srcset object will stick to the distant intrinsic original size during the first scan.

The moral here is that once published a page is displayed, do not attempt to change the size attribute. You need well for the first time, and if you are right, this means your browser resizes the assets that are best suited to the layout, i.e. the largest size finite it could be at some tell point, stop. In our example above in a vertical position above the width of the floating state.

How to specify the Sizes attributes 

A simple size attribute is the following: size = “(750px, 750px, 500px”). “That seems to media issues, said you probably against yourself. You are both good and evil. They look and work type that media issues, but they have no media issues. This is a media condition followed by something that is not CSS, but rather makes the width displayed of the image in the media state.

In our above attributes up with the browser on a screen width of 750px or smaller will be the <img> will be displayed as a 750px image (you can imagine this as a hero on mobile) and on every width of the Window above that will be displayed exactly 500px (you can view this as a profile picture). Html analyzed on the desktop and it says, “My window width is 1400px, I have a 2x screen, and the size attribute tells me that <img> will be displayed as a 500px image in media state 500px. At 2x dpi means that I have to enter at least 1000px wide version and download again. “If you have your browser not supposed to say that do not lean, I’m Anthropomorphizing.

A more complex size attribute could be represented in the form (maximum width: 479px) 96vw, (max-width: 767px) 500px; (Max-width: 991px) 48vw, 50vw. You would be able to make it even more complex by mixing in a minimal width condition, but why make life difficult. Breakpoints in your media state simply have to match the window widths that change your layout, but it is only necessary to specify some change <img> where the width value <img>.

I should stop really referring to the width of the window at this point, what I mean is the width of the window. What you see here is a mix of width px and window width. 50vw is the same as saying 50% of the width of the window. It is important to point out here that if you have a <img /> in a div, the browser to your srcset / sizes attributes deal with it analyzed the HTML code, but before it generates the page or even break down the css . The browser knows the size of the window, it has no idea of ​​the size that div will, so that all size values ​​to specify a percentage of the width of the window instead of a percentage of their parent or A relative ancestor.

Some layouts, for example, when an <img> element has multiple ancestors with different margins, you can also use calc values. In most real conditions are precise ones only wants a few bytes, while complexity increases significantly. It is important to remember that in the simplification of these complex dimensions, it is better to overestimate that underestimated.

Precision variations Responsive layouts

Imagine a similar case for a 1x screen, which specifies the size, the image is 1000px wide. If you do not do it 1000px wide variation specified in srcset, but dropped more, it is good, your browser will grasp the larger one, says a 1200px image variant and puts its intrinsic size at 1000px wide. However, if you do 1000px smaller than large variations, say that the bigger you have 800px wide, your browser will download set (the largest available) and its intrinsic size to 1000px. Your browser is fully intending to extend this thing, unless you override with CSS, but now we are in this confined space. You made a wrong turn to get here.

Do not expect your master image is just good old src attribute and is also stored in this scenario. When a browser sees that you have used a srcset and the size in an <img>, it will do so if the src cannot see it. Put this main image at the end of your list if you want srcset an option for the most recent browsers. Make sure to keep it in the src attribute for older browsers that do not see srcset / sizes.

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Uploading a larger variation than necessary will waste a few bytes of bandwidth and can be controlled by variants that match your layout as your layout is rigid enough in all media queries to support but Where size assigns a liquid size in a support request, for example: 50vw the solution is to submit to present variants that are small and wider widths to minimize bytes of spills. Generally speaking, reducing the file size of a master image to a variant such that the differences in file size between the two versions are insignificant examples of the real world. Then take a great breath and do more.

On Testing

When testing sensitive images, it is important to note that your browser has a strong cache preference. Since the point of images can react to minimize downloads to download, the browser must download a small variations of images than those it has cached.

Suppose you have an <img> element, the size attributes are set so that any more media state requires a larger version – you can do it simply by making sizes = “100%”. To open your 300px wide and slow browser to adjust the width of 2500px, since you crossed the variation width, the browser would realize that each larger is needed and you will see that the intrinsic size of the previous version is attached to the larger Variant that you need now, is downloaded and rendered. If you look at the network tab, you will see a request for each of the variants that you needed that you did. However, if you use a new cache starts 2500px wide window and gradually adjust to 300px wide, you will see only one request for the largest movement that was needed when it was created.

As with changing and testing different levels of value for sizes, set your cache again the effects of the size of the assets are less visible if you work with a larger cache instead of the smaller a user can load Your page given the viewport size for the first time with an empty cache. Eg the variant that built the intrinsic image in the memory will be different from someone with a new cache.


So the question you should ask yourself is why you need to go through all this. Naturally, the sizes of the smaller pages and reduce the loading times of your hosting bill and improve the customer experience. And sending a 5000px wide picture to a smart phone with a bad connection when it needs something tenth of the size not the best for everyone. As a means, we must strive to limit the content that we can. But there are other approaches for sensitive images that are easier to implement.

Services that give you an external script to add to your page and work directly, have less trouble getting started. But these are not intrinsically compliant standards and require the browser to make the page, java script runs and generate a reactive version of the application before the image even begins for your site visitors in some cases. In some cases, variants are generated on demand so specific that if the same client returns with a slightly varied front will inherit the force, they will generate a new version and upload, a little more or less, instead of just loading the last of the cache.

With a little work and thought you can make the experience more flexible and efficient. You can “sensitive pictures” to make a final checkbox before starting a website to reduce the work needed but ideally you want to work with reactive nature and all intrinsic size quirks and do not leave them a memory. Like forming clay or painting on a canvas you want the media what you do, you feel like creating. In Part 2, I go through the way we automate reactive images and provide real-time feedback on Webflow.

Want to work with the next generation web publishing platform? Webflow Hires!


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