var focusFirstInput = function(obj) {
	var forms = (obj && document.getElementById(obj)) ? document.getElementById(obj).getElementsByTagName('form') : document.forms;

	if (forms.length > 0 && document.forms[0].elements[0]){
		var eltype = document.forms[0].elements[0].type;
		(eltype=="text" || eltype=="textarea" || eltype=="select-one" || eltype=="select-multiple" || eltype=="password") ? forms[0].elements[0].focus() : null;
	}
}

Usage: 

//any element
window.onload = function(){
    focusFirstInput();
}

or 

// form elements under 'content-body'
window.onload = function(){
    focusFirstInput('content-body');
}

Focus first 'focus'able form field on load (DOM)

form, javascript, forms, DOM

JavaScript