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.
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.
Lighthouse also flags vendor prefix values, such as
Lighthouse flags these so that you know that you're sending more bytes in
your CSS than needed. If you're using Autoprefixer, you can prevent it
from printing out vendor prefixes by adding the following rule to your
"browserslist": [ "last 2 versions", "not ie <= 10", "not ie_mob <= 10" ]
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.