An Event Apart: Move Fast and Don’t Break Things

by Luke Wroblewski March 5, 2019

In his Move Fast and Don’t Break Things presentation at An Event Apart in Seattle, Scott Jehl shared a number of resilient patterns and tools to help us establish and maintain performant access to our Web sites. Here's my notes from his talk:

  • For successful Web design, people used to suggest we move fast and break things. Today we've become more responsible but things can still break for our users if we're not mindful.
  • So many factors that can compromise the delivery of our Web sites are out of our control. We need to be aware of these in order to build resilience into our designs.
  • We used to use browser detection and feature detection to ensure our sites were supported across Web browsers. Progressive enhancement's importance ballooned as a wide range of new devices for accessing the Web, touch interactions, and more browsers became popular.
  • Trying to make a Web site look and work the same across devices was broken, we realized this was the wrong goal and we need to adapt to varying screens, networks, input types, and more.
  • Some practices stay good. Progressive enhancement and accessibility prepared us for many of these changes but it is also a performance enhancement on its own.
  • Figuring out how to make Web sites faster used to be hard but the tools we have for measuring performance have been improving (like PageSpeedTest and WebPageTest).

Making Web Sites Fast

  • First meaningful content: how soon does a page appear to be useful to a user. Progressive enhancement is about starting with meaningful HTML and then layering additional enhancements on top of it. When browsers render HTML, they look for dependencies in the file (CSS and Javascript) before displaying anything.
  • CSS and Javascript are most often the render-blockers on sites, not images & videos. Decide if they need to load at high priority and if not, load async or defer. If you need them to run right away, consider server push (HTTP2) to send files that you know the browser needs making them ready to render right away.
  • If your server does not support push, you can inline your critical CSS and/or Javascript. Inlining however is bad for caching as it does not get reused by other pages. To get around this you can use the Cache API to inline content and cache it as a file for reuse.
  • Critical CSS tools can look over a series of files and identify the common CSS you need across a number of different pages for initial rendering. If you inline your critical CSS, you can preload the rest of your CSS (not great browser support today).
  • Inlining and push are best for first time visits, for return visits they can be wasteful. We can use cookies for checking for return visits or make use of Service Worker.
  • Time to interactive: time it takes a site to become interactive for the user. We should be aiming for interactivity in under 5seconds on a median mobile phone on 3G. Lower end phones can take a long time to process Javascript after it downloads.
  • More weight does not mean more wait. You can prioritize when things load to make pages render much faster.

Keeping Web Sites Fast

  • Making a web site fast is easier than keeping it fast. Over time, Web sites will add a number of third party services with unknown performance consequences.
  • We can use a number of tools, like Lighthouse, to track performance unfriendly dependencies. Speed Curves will let you set performance budgets and see when things are over. This allows people to ask questions about the costs of what we're adding to sites.
  • Varying content and personalization can increase optimizations but they are costly from a performance perspective since they introduce a second meaningful content render. Moving these features to the server-side can help a lot.
  • Cloudflare has a solution that allows you to manipulate pages on their server before it comes down to browser. These server-side service workers allow you to adjust pages off the client and thereby avoid delays.
  • Homepages and landing pages are often filled with big images and videos. They're difficult to keep performant because the change all the time and are often managed outside of a central CMS.
  • For really image heavy pages, we can use srcset attributes to define multiple sizes of images. Writing this markup can be tricky if written by hand. Little helper apps can allow people to write good code.
  • Soon we'll have a native lazy load feature in browsers for images and iframes. Chrome has it in testing now and can send aspect ratios before actual images.