Scale Manager
It's on the game's scale or this.scale in a scene.
Configure in the scale property of the game config.
Use scale mode FIT and skip the rest.
The scale parent is where you want to insert the game canvas.
There's no warning if you give a bad parent value, so you may need to verify once by looking in the DOM Inspector.
The Scale Manager does whole-canvas scaling or resizing, depending on the scale mode.
The scaling modes are FIT, ENVELOP, HEIGHT_CONTROLS_WIDTH, and WIDTH_CONTROLS_HEIGHT. All scaling involves some distortion, and scaling up will lose detail. Scaling means canvas pixels are mapped onto CSS pixels, e.g.,
<canvas width=100 height=100 style="width: 200px; height: 200px"></canvas>No scaling or resizing, but it's still running and emits events.
Scale to fit, meaning the largest size where both canvas dimensions don't exceed the parent. If the aspect ratios don't match, there will be gaps ("bars") one one axis.
Scale to fill, meaning the smallest size where both canvas dimensions meet the parent. If the aspect ratios don't match, the canvas will be clipped on one axis. You should ensure that only nonessential game content gets hidden.
The canvas is resized to match the parent exactly.