So it appears it does depend not only on browser, but also on operating system, right?
Yes, but also the font.
I know now XP doesn't support Unicode, what about Win7 and Win8 ??
Windows has supported Unicode since Windows NT, and characters outside of the BMP since Windows 2000, AIUI. It actually uses UTF-16 internally. XP itself should have no problem with it, but browser and operating system support aren't the only factors.
If those do support it does it mean all browsers on those systems will see my scull character on website?
All browsers on those systems will support it if the font being used contains the character. If the font doesn't contain the character, then depending on the system it may fall back on another font, or it may may show a generic "character unknown" glyph, such as the empty box. Arial Unicode MS, for example, does contain that symbol (and many other characters from the Miscellaneous Symbols block that it comes from), but Times New Roman does not (it only contains 12 characters from that block, and the Skull And Crossbones isn't one of them: white and black smiley faces, a white sun with rays, the male and female symbols, the black card suits, the eighth note and beamed eighth notes, and the musical sharp sign).
One tool you can use for checking support is FileFormat.info's Unicode tools
. It shows what ranges of Unicode characters various fonts support, and how completely. You can also test your browser's support for a block, for example the Miscellaneous Symbols block
, where you can then click on a character to see various properties, including a link to a list of fonts (that FileFormat.info knows about) that contain the character.