What are CSS custom properties?
PICTURE THIS: HOW TO EXPLAIN IT
Simple meaning
They are variables defined with a double-dash name, often on :root, and read with var.
WHY — CSS instead of guessing?
Why interviewers care about CSS:
who only read docs from people who shipped.
and tied to Frontend work.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1They are variables defined
with a double-dash name, often on :root, and read with var.
- 2They cascade and can
be changed at runtime for theming.
- 3Unlike Sass variables they
exist in the browser, so I can toggle dark mode without rebuilding CSS.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
They are variables defined with a double-dash name, often on :root, and read with var. They cascade and can be changed at runtime for theming.