How to limit x-axis rotation of camera

What would be the simplest way to limit the rotation of the x-axis of my camera. I am trying to avoid using GetAxis and the mouse for moving the camera. And I want to avoid doing a an inside or outside loop when trying to look at the floor or ceiling. The biggest problem I'm having is the an Euler snaps back to zero.

Right now, learning the correct method to limit the x-rotation is more important than cleaning up the code, which I will do down the road.

  using UnityEngine;
     using System.Collections;
     
     public class Movement : MonoBehaviour 
     {    
         public int rotateSpeed;
         public int movementSpeed;
             
         void FixedUpdate()
         {    
             //Move player object forward along local z-axis
             if(Input.GetKey(KeyCode.W))
             {
                 transform.Translate(Vector3.forward * Time.deltaTime * movementSpeed);
             }
             
             //Move player object backward along local z-axis
             if(Input.GetKey(KeyCode.S))
             {
                 transform.Translate(Vector3.back * Time.deltaTime * movementSpeed);
             }
             
             //Strafe left
             if(Input.GetKey(KeyCode.A))
             {
                 transform.Translate(Vector3.left * Time.deltaTime * movementSpeed);
             }
             
             //Strafe right
             if(Input.GetKey(KeyCode.D))
             {
                 transform.Translate(Vector3.right * Time.deltaTime * movementSpeed);
             }
             
             //Rotate/Look player up
             if(Input.GetKey(KeyCode.UpArrow))
             {
                 transform.Rotate(Vector3.left * Time.deltaTime * rotateSpeed);
             }
             
             //Rotate player down
             if(Input.GetKey(KeyCode.DownArrow))
             {
                 transform.Rotate(Vector3.right * Time.deltaTime * rotateSpeed);
             }
             
             //Rotate player left
             if(Input.GetKey(KeyCode.RightArrow))
             {
                 transform.Rotate(Vector3.up * Time.deltaTime * rotateSpeed, Space.World);
             }
             
             //Rotate player right
             if(Input.GetKey(KeyCode.LeftArrow))
             {
                 transform.Rotate(Vector3.down * Time.deltaTime * rotateSpeed, Space.World);
             }
         }
         
     }