Why is this shader using multiple render targets not working?

I have a webgl shader that uses multiple render targets. This one only renders to one texture for now. However, I plan that this shader shall render more. However, as soon as I add layout qualifiers, and #version 300 es, my shader stops working, because I get a message saying it is an invalid program.

  #version 300 es

  uniform mat4 viewProjectionMatrix;

  attribute vec3 vertex;
  attribute vec2 uv;

  varying vec2 vUv;

  void main() {
    gl_Position = viewProjectionMatrix * vec4(vertex, 1);
    vUv = uv;
  }

Fragment shader

  #version 300 es

  precision mediump float;

  layout(location = 0) out vec4 color;

  uniform sampler2D textureId;

  varying vec2 vUv;

  void main() {
    color = texture2D(textureId, vUv);
  }