Handler for bidirectional unit conversion form
Instead of inlining the script, use addEventListener instead. To retrieve the elements, you'd need something like getElementsByClassName or querySelectorAll to return an array-like structure of elements. You loop through them, and attach a listener each.

var formInputs = Array.prototype.slice.call(document.querySelectorAll('.form-input'));
formInputs.forEach(function(element){
  element.addEventListener('blur', function(){
    // blur code
  });
  element.addEventListener('keyup', function(){
    // blur code
  });
});


The Array.prototype.slice.call is to convert the array-like structure returned by querySelectorAll into a real array. That's so I can use forEach. But a regular for loop will do, since a NodeList (the array-like structure) has a length property.

Now instead of alert and return, why not throw an error instead?

if(isNaN(value)) throw new Error('Non-numeric value');


Also, instead of putting all your conversions in a single function and sending a "sense", why not create a function for each kind of conversion? It's more verbose, but you don't have to pile up everything in one function.