function checkFields(){
if(document.getElementById('naam').value != '' && document.getElementById('mail').value != '' && document.getElementById('onderwerp').value != '' && document.getElementById('bericht').value != ''){
return true;
}

else{
if(document.getElementById('naam').value == ''){
document.getElementById('naam').style.borderColor = 'red';
if(window.addEventListener){
document.getElementById('naam').addEventListener('focus',function(){this.style.borderColor = '#999';},false);
}else{
document.getElementById('naam').onfocus = function(){this.style.borderColor = '#999';};
}
}
if(document.getElementById('mail').value == ''){
document.getElementById('mail').style.borderColor = 'red';
if(window.addEventListener){
document.getElementById('mail').addEventListener('focus',function(){this.style.borderColor = '#999';},false);
}
else{
document.getElementById('mail').onfocus = function(){this.style.borderColor = '#999';};
}
}
if(document.getElementById('onderwerp').value == ''){
document.getElementById('onderwerp').style.borderColor = 'red';
if(window.addEventListener){
document.getElementById('onderwerp').addEventListener('focus',function(){this.style.borderColor = '#999';},false);
}else{
document.getElementById('onderwerp').onfocus = function(){this.style.borderColor = '#999';};
}
}
if(document.getElementById('bericht').value == ''){
document.getElementById('bericht').style.borderColor = 'red';
if(window.addEventListener){
document.getElementById('bericht').addEventListener('focus',function(){this.style.borderColor = '#999';},false);
}else{
document.getElementById('bericht').onfocus = function(){this.style.borderColor = '#999';};
}
}
alert('U heeft niet alle velden ingevuld, vul de rood omkaderde velden in en klik op \"Verzenden\".');
return false;
}
}