9.99 rounded to 2 sig figs is 10., that's ten with a period

It won't be 10, that's ten without the period.

Notice the difference between 10. and 10, where this is a period and where there isn't. The period, aka decimal point, indicates that the zero is a significant digit. Therefore, 10 has one signifcant digit and 10. has two significant digits. You can also indicate the significant zero with a bar (a short line) written over it, or you can always put it in scientific notation. That is the most fool-proof. 1.0x10^1 = 10., which is ten with a decimal, all of which has two significant digits.