The title says it all. What the heck is it?
Edit: I found this on my car, in Kansas city.
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After conducting a small investigation, it would appear most likely that the spider captured in the attached photograph is an arachnid in the family 'Thomsidae' or 'Crab Spiders'.
Although the proposed genus of Araniella proposed above by user137 shares a superficial similarity to this family, thomsids generally do not construct webs to ensnare prey - I might hazard that the web on your car does not hold the same structure, as shown below, to that of an Orb Weaver Spider (Araniella).
From the available information it is challenging to propose a genus, let alone a species. Based on the location of the car, I may propose the sub-family Misumenoides or genus Thomisus.
The general characteristics of crab spiders are the ambush of prey as opposed to trapping prey; the inverted V-shape under the cephalothorax; and the sideways movement of certain species in the Thomsidae family. It is of interest to note that any member in the population could vary appreciably in colour.
Wikipedia article concerning Crab Spiders available from: https://en.wikipedia.org/wiki/Thomisidae Britannica regarding Crab Spiders available from: https://www.britannica.com/animal/crab-spider Information regarding Misumenoides is available from: https://en.wikipedia.org/wiki/Misumenoides