Renderle

The daily email CSS game

Email developers spend their lives guessing which client will break what. Renderle turns that into a game. Five features, five clients, one 5x5 grid, a new puzzle every day at midnight UTC. Answers come from the same support matrix behind the Emailens CSS reference, so a wrong guess is worth something: it is a rendering trap you now know about.

It is not only properties. The pool is 213 features that email clients genuinely disagree about, spread across 6 categories, and every card tells you which one you are looking at.

• CSS property144
• selector26
• HTML element25
• CSS unit7
• pseudo-element6
• CSS at-rule5

How it works

1. 01One feature at a timeRenderle deals 5 features a day. Each card names a CSS property, selector, at-rule, unit or HTML element.
2. 02Answer client by clientFor each of the five clients, pick No, Partial or Yes. Answers reveal as you tap, so you learn the row before you finish it.
3. 03Collect the gridEvery answer becomes a square. Green is exact, yellow is one step off, red is two. Twenty five squares, one score.
4. 04Come back tomorrowThe puzzle rolls over at midnight UTC. Finish two days running and your streak starts counting.

The five clients

• GmailFull support table
• Outlook (New)Full support table
• Apple MailFull support table
• Yahoo MailFull support table
• Samsung MailFull support table

Questions

What is Renderle?

Renderle is a daily guessing game for email developers. Each day you get five CSS or HTML features and, for each one, you say whether it renders in five email clients: No, Partial or Yes. Every answer reveals instantly, and a finished day gives you a 5x5 grid you can share.

How often does the puzzle change?

Once a day, at midnight UTC. Everyone playing on the same UTC day gets the same five features in the same order, which is what makes the shared grids comparable.

Which email clients does Renderle use?

Five: Gmail, Outlook (New), Apple Mail, Yahoo Mail, Samsung Mail. They are picked to span the rendering engines that disagree most, so a feature that behaves the same everywhere never becomes a puzzle.

What kinds of features show up?

Not just CSS properties. The pool is 213 features across 6 categories: 144 CSS properties, 26 selectors, 25 HTML elements, 7 CSS units, 6 pseudo-elements, 5 CSS at-rules. Every card is labelled with its category, so you always know whether you are being asked about a property, a selector, an at-rule, a unit or an element.

Where do the answers come from?

From the same support matrix behind the Emailens CSS reference, derived from caniemail.com (MIT) and carried in the @emailens/engine package, data version 0.10.4, last verified Apr 27, 2026. If you disagree with an answer, the feature page shows the underlying data.

How is the score calculated?

Out of 25, one point per exactly correct answer. In the shared grid a green square is an exact match, yellow is one step off, for example guessing Yes where the real answer is Partial, and red is two steps off. Only greens count toward the score.

Do I need an account to play?

No. Progress, streak and best streak live in your browser's local storage. There is nothing to sign up for and nothing is sent to a server.