Why the audit is important
The old, 2009 specification for Flexbox is deprecated and is 2.3x slower than the latest specification. See Flexbox Layout Isn't Slow to learn more.
How to pass the audit
Under URLs, Lighthouse lists every instance of
display: box that it found
on your page's stylesheets. Replace every instance with the new syntax,
If a stylesheet is using
display: box, then it may be using other deprecated
Flexbox properties. In short, every property that begins with
box-flex, is deprecated and should be replaced. See
CSS Flexbox 2009/2011 Spec Syntax Property Mapping to see exactly how the
old properties map to the new ones.
How the audit is implemented
This section explains how this audit is implemented, so that you can understand how the audit's score is calculated.
Lighthouse collects all of the stylesheets used on the page and checks if any of
display: box. Lighthouse does not check if the stylesheets use any
other deprecated properties.