Assuming it's a Marketo form on a Marketo landing page, there should be a non-javascript dependent alternative form which loads if the lead has javascript disabled. Is there a particularly large volume of leads with js disabled going to your website? My understanding is that less than .5% of internet users browse with javascript disabled - really it's almost impossible to use the internet much without it. If you're seeing forms break for 25% of your leads then it sounds like something else is wrong, perhaps there's a js conflict happening.