First off, you wrote defD as defF in your code by accident.
But more importantly, you are mixing strings and numbers. In Javascript, innerHTML returns a string. A string can be any piece of text, like "I am a jelly donut". It returns a string because innerHTML is capable of carrying more than numbers - it could contain text too. So innerHTML returns a string, just to be safe.
As such, you are trying to add a number to a string (piece of text) and you can't add a number to a sentence. So Javascript decides to treat the number like another piece of text rather than a number, and simply tacks the second number on the end of the first one rather than doing a mathematical equation.
Now many others have been saying "use parseInt, use parseInt!" and normally, that would work. That is because parseInt is a function that takes strings and converts them to numbers. However, when you start out, some of your textboxes are empty, so parseInt does not know what to do. So you get NaN (or, "Not a Number" to be exact) because the box is blank.
Usually, I avoid using parseInt because it is a function and in general, using plain math works faster and better than using a function in JS. An easy workaround to your problem is to to multiply the strings by one. Now, I know what you're thinking. "But if adding a number to text doesn't work, why does multiplying?" Simple. You cannot multiply text. As such, Javascript is forced to think of it like two numbers, rather than two strings.
atk += document.getElementById('atk').value*1;
spa += document.getElementById('spa').value*1;
def += document.getElementById('def').value*1;
spd += document.getElementById('spd').value*1;
hp += document.getElementById('hp').value*1;
spe += document.getElementById('spe').value*1;
document.getElementById("atkD").innerHTML = atk;
document.getElementById("spaD").innerHTML = spa;
document.getElementById("defD").innerHTML = def;
document.getElementById("spdD").innerHTML = spd;
document.getElementById("hpD").innerHTML = hp;
document.getElementById("speD").innerHTML = spe;
An added bonus is that this won't return NaN like parseInt does. Try it in your code and see.
(Note: you might want to use a for loop to loop through those and shorten your code instead. It isn't really necessary, but it would look nicer in your code.)
