Note: This feature is available in Web Workers.
The read-only WebGLShaderPrecisionFormat.rangeMax property returns the base 2 log of the absolute value of the maximum value that can be represented.
const canvas = document.getElementById("canvas");
const gl = canvas.getContext("webgl");
gl.getShaderPrecisionFormat(gl.VERTEX_SHADER, gl.MEDIUM_FLOAT).rangeMax; // 127
gl.getShaderPrecisionFormat(gl.FRAGMENT_SHADER, gl.LOW_INT).rangeMax; // 24