CL https://golang.org/cl/13882 mentions this issue.

Implementing these interfaces is less trivial than it seems.

Fast and looseless implementation could use hexadecimal notation with 'p' (binary) exponent. But it depends about the expectations of MarshalText: Should it always return a string representation that is a "standard" floating-point number that might be read by another system? In that case, the 'p' notation might not be option.

An implementation that is more similar to float32/64 can use 'g' with precision -1 (once implemented) but it's a very expensive conversion. Additionally currently there is no way to parse a string without loosing precision. Parse's semantic may actually not be correct as is: It perhaps should parse w/o rounding error if the precision is 0.

Decided to keep it simple for now and implement this simply via the 'g' with precision -1 format. Also, no recording of precision or accuracy. This makes this work for json w/o trouble.

CL https://golang.org/cl/15040 mentions this issue.