What to ask your agency about critical CSS

It comes up on almost every project, usually later than it should. If you are briefing an agency or a freelancer on critical CSS, these questions are worth asking early.

Speed is a feature people notice only in its absence, and then they leave rather than complain. Assume whoever inherits this will have half your context and none of your patience.

Questions worth asking

• Who will actually do this work, and have they done it before?
• How will we know afterwards whether it worked?
• What happens if it needs changing in a year?
• What are you assuming that we have not confirmed?

What a good answer sounds like

Inlining what is needed first shortens time to first paint. That sounds obvious written down. It is still the thing most often skipped. Assume whoever inherits this will have half your context and none of your patience.

Only worth the complexity on high-traffic pages. In practice this is a scheduling problem more than a technical one. The teams that stay on top of it are the ones who put it on a calendar rather than a wish list.

How to tell if yours is fine

Real-world numbers from actual visitors matter more than a score produced on a fast laptop. Three things worth confirming about critical CSS before you move on:

• Someone can say what the current setup is without going to look
• Only worth the complexity on high-traffic pages — and you know whether that is true here
• There is a way to tell whether the last change to this helped

If you are not sure where your systems currently stand on this, it takes us about an hour to find out.