Designing an HTML5 Game That Doesn't Break on Half Your Players' Screens

A game built and tested on one 16:9 desktop monitor will meet an enormous spread of real screens the moment it goes live — tall phone aspect ratios, tablets, ultrawide monitors, browser windows resized to a quarter of the screen. None of those are edge cases; together they're most of your actual traffic. Designing for a single fixed resolution and hoping the rest scales gracefully is the single most common cause of "the game looks broken" reports that have nothing to do with the game logic itself.

Pick a scaling strategy on purpose, not by accident

The three common approaches are: stretch-to-fit (the whole canvas scales to the container, distorting non-matching aspect ratios), letterbox/pillarbox (keep the original aspect ratio and add bars on the sides or top/bottom), and dynamic viewport (the play area itself expands or contracts to fill available space, showing more or less of the world rather than distorting it). Stretching is the easiest to implement and the worst-looking; dynamic viewport takes the most design work but holds up best across the full range of real screens.

Design the UI and the play area as separate problems

• HUD elements anchored to screen edges (score, timer, buttons) should stay a fixed distance from the edge across resolutions, not scale proportionally with the play area — proportional scaling makes UI either tiny on large screens or huge on small ones.
• Keep critical gameplay elements away from the extreme edges of the canvas. A tall phone screen may crop what a wide desktop window shows in full.
• Test your UI at the narrowest realistic width before the widest — it's much easier to notice cramped UI than to notice UI floating awkwardly in extra space.
• If your game supports both portrait and landscape, treat them as genuinely different layouts, not one layout that just gets taller or wider. Control placement especially rarely transfers cleanly between the two.

Aspect ratio breaks games differently depending on genre

A puzzle or match-3 game with a fixed, centered board tolerates aspect ratio changes well — extra space just becomes background. A platformer or shooter where the player needs to see threats coming from off-screen tolerates it badly — a narrower viewport on one aspect ratio can hide an obstacle that was clearly visible in testing on a wider one. Know which category your game falls into, and test the narrow case specifically if it's the second one.

Test on real breakpoints, not just a resized browser window

A desktop browser resized smaller doesn't perfectly reproduce a real phone's viewport, pixel density, or the way mobile browser chrome eats into visible height. Where possible, test on at least one real phone and one real tablet in both orientations before submission — several layout bugs only show up with a browser's actual mobile rendering behavior, not its resized desktop approximation of it.

Touch-specific control placement is covered separately in building for mobile web — that guide and this one are meant to be read together, since layout and control design on mobile are two halves of the same problem. See PlayKovo's submission rules for the minimum supported resolutions and orientations expected at review.