<form id="loginForm" onsubmit="return loginValidation()>
 <p class="error" id="liusrError"></p>
 <label for="usr">Username :</label>
 <br />
 <input type="text" name="liusr" id="liusr">
 <br />
 <p class="error" id="lipwdError"></p>
 <label for="pwd">Password :</label> <br />
 <input type="password" name="lipwd" id="lipwd">
 <br />
 <a id="fgtpwd" href="url">Forgot your password?</a>
 <br />
 <input type="submit" value="Log In" id="logIn">
</form>

function loginValidation(){
 var pwd = document.getElementById("lipwd").value;
 var usr = document.getElementById("liusr").value;
 var error = false;

 if(pwd == null || pwd == ""){
  document.getElementById("lipwdError").innerHTML = "Please enter a valid password";
  error = true;
 }else{
  document.getElementById("lipwdError").innerHTML = " ";
 }

 if(usr == null || usr==""){
  document.getElementById("liusrError").innerHTML = "Please enter a valid username";
  error = true;
 }else{
  document.getElementById("liusrError").innerHTML = " ";
 }

 return error;
}

<form id="loginForm" onsubmit="return loginValidation()"
    method="POST" action="/login">

onsubmit="return yourfunction()"

<form id="loginForm" onsubmit="return loginValidation()>