Unity Shader Error when Building to WebGL - unity3d

Question

When trying to build my game to Webgl I receive the following errors:
Shader error in 'Hidden/Universal/CoreBlit': invalid subscript 'positionCS' at /PathToProject/Library/PackageCache/com.unity.render-pipelines.core#12.1.2/Runtime/Utilities/Blit.hlsl(92) (on gles)
Shader error in 'Hidden/kMotion/CameraMotionVectors': SV_VertexID semantic is not supported on GLES 2.0 at line 11 (on gles)
The Shader I am using has the following code:
Shader "Skybox Gradient"
{
Properties
{
_Top("Top", Color) = (1,1,1,0)
_Bottom("Bottom", Color) = (0,0,0,0)
_mult("mult", Float) = 1
_pwer("pwer", Float) = 1
[Toggle(_SCREENSPACE_ON)] _Screenspace("Screen space", Float) = 0
}
SubShader
{
Tags { "RenderType"="Opaque" }
LOD 100
CGINCLUDE
#pragma target 3.0
ENDCG
Blend Off
Cull Back
ColorMask RGBA
ZWrite On
ZTest LEqual
Offset 0 , 0
Pass
{
Name "Unlit"
Tags { "LightMode"="ForwardBase" }
CGPROGRAM
#ifndef UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX
//only defining to not throw compilation error over Unity 5.5
#define UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(input)
#endif
#pragma vertex vert
#pragma fragment frag
#pragma multi_compile_instancing
#include "UnityCG.cginc"
#pragma shader_feature_local _SCREENSPACE_ON
struct appdata
{
float4 vertex : POSITION;
float4 color : COLOR;
UNITY_VERTEX_INPUT_INSTANCE_ID
};
struct v2f
{
float4 vertex : SV_POSITION;
#ifdef ASE_NEEDS_FRAG_WORLD_POSITION
float3 worldPos : TEXCOORD0;
#endif
UNITY_VERTEX_INPUT_INSTANCE_ID
UNITY_VERTEX_OUTPUT_STEREO
float4 ase_texcoord1 : TEXCOORD1;
float4 ase_texcoord2 : TEXCOORD2;
};
uniform float4 _Bottom;
uniform float4 _Top;
uniform float _mult;
uniform float _pwer;
v2f vert ( appdata v )
{
v2f o;
UNITY_SETUP_INSTANCE_ID(v);
UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(o);
UNITY_TRANSFER_INSTANCE_ID(v, o);
float4 ase_clipPos = UnityObjectToClipPos(v.vertex);
float4 screenPos = ComputeScreenPos(ase_clipPos);
o.ase_texcoord2 = screenPos;
o.ase_texcoord1 = v.vertex;
float3 vertexValue = float3(0, 0, 0);
#if ASE_ABSOLUTE_VERTEX_POS
vertexValue = v.vertex.xyz;
#endif
vertexValue = vertexValue;
#if ASE_ABSOLUTE_VERTEX_POS
v.vertex.xyz = vertexValue;
#else
v.vertex.xyz += vertexValue;
#endif
o.vertex = UnityObjectToClipPos(v.vertex);
#ifdef ASE_NEEDS_FRAG_WORLD_POSITION
o.worldPos = mul(unity_ObjectToWorld, v.vertex).xyz;
#endif
return o;
}
fixed4 frag (v2f i ) : SV_Target
{
UNITY_SETUP_INSTANCE_ID(i);
UNITY_SETUP_STEREO_EYE_INDEX_POST_VERTEX(i);
fixed4 finalColor;
#ifdef ASE_NEEDS_FRAG_WORLD_POSITION
float3 WorldPosition = i.worldPos;
#endif
float4 screenPos = i.ase_texcoord2;
float4 ase_screenPosNorm = screenPos / screenPos.w;
ase_screenPosNorm.z = ( UNITY_NEAR_CLIP_VALUE >= 0 ) ? ase_screenPosNorm.z : ase_screenPosNorm.z * 0.5 + 0.5;
#ifdef _SCREENSPACE_ON
float staticSwitch13 = ase_screenPosNorm.y;
#else
float staticSwitch13 = i.ase_texcoord1.xyz.y;
#endif
float4 lerpResult3 = lerp( _Bottom , _Top , pow( saturate( ( staticSwitch13 * _mult ) ) , _pwer ));
finalColor = lerpResult3;
return finalColor;
}
ENDCG
}
}
CustomEditor "ASEMaterialInspector"
}
The error only occurs every second or third time I try to locally build the game.
However, when I am building with Unity Cloud Build it always appears.