Critical CSS: a practical guide

It is rarely the thing that gets a project approved, and often the thing that decides how it goes. This guide covers what critical CSS actually involves, where it usually goes wrong, and how to tell whether yours is in reasonable shape.

Speed is a feature people notice only in its absence, and then they leave rather than complain. Nothing below assumes a large team or a large budget — most of it is a decision somebody has to make and then write down.

What is actually at stake

Inlining what is needed first shortens time to first paint. It is worth being explicit about, because assumptions differ quietly. The failure mode is not doing it wrong, it is doing it once and assuming it stays done.

For most businesses the question is not whether this matters but how much of it is worth doing right now. That depends on what you are trying to achieve in the next few months, not on best practice in the abstract. The cost of getting this wrong is rarely visible on the day it happens.

The practical version

The rest can load without blocking. The reasoning matters more than the rule, because the rule has exceptions. The teams that stay on top of it are the ones who put it on a calendar rather than a wish list.

Real-world numbers from actual visitors matter more than a score produced on a fast laptop. The version that works in practice is usually less elaborate than the version described in the guides.

Only worth the complexity on high-traffic pages. The reasoning matters more than the rule, because the rule has exceptions. It rarely shows up as a line item, which is exactly why it slips.

A working checklist

If you want a quick read on where you stand, work through this. Anything you cannot answer confidently is where to start.

• Inlining what is needed first shortens time to first paint
• The rest can load without blocking
• Only worth the complexity on high-traffic pages
• Someone is named as the owner, not just assumed to be
• There is a date in the calendar to review it again
• The decision and the reasoning behind it are written down somewhere findable
• You could explain the current setup to a new hire in five minutes

Where it usually goes wrong

The most common failure is not doing this badly. It is doing it once, during a launch, and never revisiting it. Circumstances move, the setup does not, and the gap widens quietly until something breaks or somebody notices the numbers.

• It was configured during a launch and has not been touched since
• Different people in the business believe different things are true about it
• There is no way to tell whether the last change helped or hurt
• The only person who understands it has left, or is about to

Performance work is mostly subtraction, which makes it unpopular and effective. Getting it slightly wrong is survivable. Ignoring it entirely is not.

How we approach it

On our projects this gets handled during the build rather than added afterwards, because retrofitting it costs several times more than including it. We write down what was decided and why, so the next person to touch it is not guessing.

If you are working with someone else, the questions worth asking are simple: who owns this, how will we know it is working, and what happens when it needs to change?

Making it stick

Pick the single item from the checklist above that would cause the most trouble if it turned out to be wrong. Fix that one, confirm it worked, then move on. None of this needs a rewrite. Most of it is a morning's work once someone decides to do it.