> Browsers accept 2147483648

okay 50000 then (to save few bytes :)

> to ensure no CSS conflicts is to declare the properties in a stylesheet, flagged !important (this option seems worse to me).

What conflicts you are expecting? I'm not sure those conflicts will ever exist. If they will be what stopping the person who creates those conflicts to use !important? You can add e.g. data-grab-overlay attribute to increase the CSS specifity, if you want, but I don't think it's needed.