Specification: http://url.spec.whatwg.org/#concept-ipv6-serializer

Current algorithm may add extra colons.

For example 1:2::7:8
pieces = [1, 2, 0, 0, 0, 0, 7, 8]

In this case, after output 2, it outputs : and then output :: with compress pointer like "1:2:::7:8".
To fix this, the algorithm should be changed like following:

4. Let skipflag be false.
5. For each piece in address's pieces, run these substeps:
5.1. If compress pointer points to piece, append "::" to output, let skipflag be true, and then skip subsequent substeps.
5.2. If skipflag is true, run these substeps:
5.2.1. If piece is 0, and then jump to Main.
5.2.2. let skipflag be false.
5.3. Otherwise, if piece is not the first of address's pieces, append ":" to output.
5.4. Append piece, represented as the shortest possible lowercase hexadecimal number, to output.