Given that attributes are provided as inputs to shaders, in GLSL 300 ES, the attribute qualifier is removed. For example, with WebGL's GLSL 100, you might have the following:

attribute vec3 aVertexNormal;
attribute vec4 aVertexPosition;

In GLSL 300 ES, this would be as follows:

in vec3 aVertexNormal;
in vec4 aVertexPosition;