In WebGL you pass a format/type pair to readPixels. For a given texture internal format (attached to a framebuffer), only 2 combinations of format/type are valid.

From the spec:

For normalized fixed-point rendering surfaces, the combination format RGBA and type UNSIGNED_BYTE is accepted. For signed integer rendering surfaces, the combination format RGBA_INTEGER and type INT is accepted. For unsigned integer rendering surfaces, the combination format RGBA_INTEGER and type UNSIGNED_INT is accepted.

The second combination is implementation defined which probably means you shouldn't use it in WebGL if you want your code to be portable. You can ask what the format/type combination is by querying

// assuming a framebuffer is bound with the texture to read attached
const format = gl.getParameter(gl.IMPLEMENTATION_COLOR_READ_FORMAT);
const type = gl.getParameter(gl.IMPLEMENTATION_COLOR_READ_TYPE);

Also note what texture formats that are renderable, meaning you can attach them to a framebuffer and render to them, are also somewhat implementation defined. WebGL1 requires only one combination to be renderable, RGBA/UNSIGNED_BYTE. All others are optional (LUMINANCE for example) and some are can be made renderable by extension, RGBA/FLOAT for example.

The table below is live. You may notice that it gives different results depending on the machine, OS, GPU, or even browser. I know on my machine Chrome and Firefox give different results for some of the implementation defined values.