Simple JavaScript library for DOM interactions
First off, consider which browsers is this DOM library supposed to support. Then test that it works in all of them.


create()


Plain for-in loop is not safe as it also loops over keys in object prototype.
temp is bad name for a variable.
You're repeatedly testing that return value is window.Element.
window.Element can be written as just Element (or perhaps use HTMLElement for clarity).

attr()


What if the value is 0 or ""?

prepend()


elem.prepend() is not supported by all browsers.

remove()


par is unnecessary abbreviation of parent.



A few general remarks:


Check your code for consistent formatting. You're missing some semicolons and the indentation is inconsistent. Use a tool like ESLint.
Escaping newlines to create multiline strings is not officially supported in ECMAScript.