swap - show text as soon as possible, and always swap in the web font when it loads.block - hide text for up to three seconds while waiting for the web font, and always swap in the web font when it loads.There are other options, shown in the image below: This means that on uncached page loads there is a chance that the fallback font will be used, but all subsequent page loads should be rendered with the web font, as it will be downloaded and available in cache. This directive lives in your font-face declaration and tells the browser to use a fallback system font if the web font is not available at the time of rendering text (plus 100ms). The dirty little secret of this blog post is that you can resolve layout shifts due to fonts with a single line of CSS: Prevent layout shifts with font-display # I've covered general layout shifts in more detail in a post on How to Improve Core Web Vitals. Chrome developer tools shows attribution of layout shifts You can also find layout shifts in the Chrome Developer Tools Performance tab, look for layout shifts attributed to text elements. Headline web font loading causes a layout shift. Layout shifts are dependent on viewport size so make sure you test a few different devices. The Washington Post has a couple of layout shifts due to web fonts, in the example below the headline text is one line shorter with the web font than without which results in a layout shift. There is a toggle to show layout shifts, combine that with 'huge' screenshot images and a 0.1s interval to get a clear view of what is happening as your page renders (you can simply add this to the end of the URL: &highlightCLS=1&thumbSize=600&ival=100&end=visual&text=000&bg=fff"). There are a number of methods to detect layout shifts due to web fonts, the most simple is to run your page through WebPageTest and use the filmstrip view. Flash of Unstyled Text Detecting layout shifts # Fusce in maximus ante, non malesuada justo. Vestibulum tempus diam vel ex venenatis placerat. Nam ultrices rhoncus dui, at pretium erat rhoncus et. Maecenas rhoncus faucibus dui quis pretium. Praesent feugiat consequat risus, ut eleifend neque consequat quis. Mauris cursus diam id ipsum aliquet tempus. Morbi vel dui non risus aliquam rhoncus in ac justo. Suspendisse quis elit fringilla, luctus felis ut, ultricies leo. Proin aliquam metus eu lacus placerat feugiat. Lorem ipsum dolor sit amet, consectetur adipiscing elit. When laying out a page, browsers will use the dimensions and properties of the fallback font to determine the size of the containing elements, even if you have declared a web font to block the system font with font-display: block! This occurs when the height of the font or the length of the paragraph is different with the web font compared to the system font. a or paragraph) changes when the web font is downloaded. Fonts cause layout shifts when the size of the containing element (e.g. Unexpected layout shifts (page content moving around without user interaction) are bad for user experience.
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