From Restarters Wiki

All still have formatting problems, with text overflowing onto subsequent paragraphs and/or covering the icon.

My preference is for the last icon. Amber is a good colour for a warning, and an exclamation mark is more direct and less open to differing interpretations than a stroke of lightning. (I have an issue with Apple's and Microsoft's over-reliance on icons. It's not true that a picture is worth a thousand words if the meaning of the picture is unclear or open to different interpretations. If I say "WARNING" in big letters, it's clear that I mean what I say and I say what I mean. A symbol just serves to draw the eye to it. </rant>

For consistency, the size of the symbol shouldn't change otherwise you'll get a tiny one on one page and a humungous one on another, relating more to the verbosity of the author than the severity of the warning.

To my eye and having experimented a bit, the ideal symbol height is around 3 lines of text.

- Philip