Basically, lazy loading is a technique for late loading of the images, video or iframe resources used on the pages, out of the visible screen, to increase the performance of web pages.
Each of the resources used in web pages creates a load on the page, according to HTTPArchive images are the most popular resource on websites. Since they use more images, they use more bandwidth than other sources.
Over the years, the load of images used on the pages has been increasing.
As the load on the page increases, the performance of the web pages is affected by this situation, loading only the images that appear on the screen at the time of loading the page and delaying the invisible images always contribute to data savings and performance gain.
Google’s Page Speed Insight Tool and some third-party tools warn that off-screen images should be delayed on websites.
Warning to delay loading of off-screen images on Google Page Speed Insight
Video elements such as visual elements can also be delayed. Videos are usually uploaded with the
<video> tag. (In some cases, there are cases where
<img> tags are used.)
To delay videos,
preload="none" is added to the video tag to prevent preloading of video data. Although it varies according to the browsers, since the preload feature is specified as auto in some browsers by default, video data can be preloaded, to prevent this, the
preload feature is marked as none.
For the videos that will be started with the click of the user, the preview image is indicated by the
poster feature until the video is uploaded.
<video controls preload="none" poster="preview.jpg"> <source src="..." type="video/webm"> <source src="..." type="video/mp4"> </video>
Along with the native lazy loading
loading="lazy" feature that comes with the Chrome 77 update released on September 16, 2019, images (
<img>) and iframe (
<iframe>) tags are delayed.
It is seen that the usage rate of native lazy loading has increased over time.
In the delay method implemented with the
Off-screen images are delayed with the following usage, which comes with Chrome 77 and is also applied in later versions.
<img src="..." alt="..." loading="lazy" width:"..." height:"...">
Loading feature has 3 supported values;
NOT: Although valid in Chromium, the auto feature is not mentioned on whatwg.org. It is not recommended to be used without passing it here, it is not a feature of extra importance anyway.
In addition, the delay feature works on images defined with the
<picture> <source media="(min-width: 800px)" srcset="large.jpg 1x, larger.jpg 2x"> <img src="..." alt="..." loading="lazy" width="..." height="..."> </picture>
In this structure, all that needs to be done to delay the image to be captured in the
<source> element is to use the
loading="lazy" attribute within the
Note: Native lazy-loading does not work on background images used with CSS, it only works with
loading="lazy"in iFrame Tags
Delaying iFrame tags is used in Chromium base.
<iframe src="..." loading="lazy" width="..." height="...">
iFrame lag works differently than visual lags. It varies depending on whether the iFrame tags are hidden or not. (Hidden iFrame tags are often used for analytical purposes.)
Chrome considers the following criteria when determining whether an iFrame tag is hidden;
When these situations apply, Chrome considers the iFrame tag to be hidden and does not delay the resource in most cases.
Compatible with Chromium supported browsers. Browsers that are not compatible with the Loading attribute will ignore it without any side effects.
Note: These data may change in the future, visit https://caniuse.com/loading-lazy-attr for updated data!
Browsers don’t know these dimensions right away unless the widths and heights of the images are explicitly specified.
The browser needs to know the widths and heights of the images so that they can allocate space for the images to be loaded on the pages. Therefore, the width and height values of the images should be specified.
When image sizes are not specified, it can cause layout shifts, which is more noticeable on pages with a long load time.
<img src="..." alt="..." loading="lazy" width="300" height="200">
Alternatively, it can be used as inline CSS, but inline CSS is not recommended for SEO.
<img src="..." alt="..." loading="lazy" style="width:300px; height:200px;">
It is important to specify the dimensions of the images, with or without the use of lazy load. This situation becomes a little more important when using lazy loading. Specifying the width and height values also allows modern browsers to calculate the image size on the page.
Most of the time, lazy loading works even if the dimensions of the images are not specified, but in some cases it may not work.
In a case where no image dimensions are specified, the initial dimensions of the images are 0 to 0. In this way, on a page with unspecified images, the browser may decide to load everything, calculating that all the content will fit on the screen and that the user will see all the content.
Specifying the size of images reduces the occurrence of cumulative layout shifts. If the image dimensions are not provided somehow, lazy loading can create a balance between saving network resources and risking layout shift.
The Lazy Loading feature in WordPress infrastructure comes by default with WordPress version 5.5.