Awesome
Parse 
This package contains several lexers and parsers written in Go. All subpackages are built to be streaming, high performance and to be in accordance with the official (latest) specifications.
The lexers are implemented using buffer.Lexer in https://github.com/tdewolff/parse/buffer and the parsers work on top of the lexers. Some subpackages have hashes defined (using Hasher) that speed up common byte-slice comparisons.
Buffer
Reader
Reader is a wrapper around a []byte that implements the io.Reader interface. It is comparable to bytes.Reader but has slightly different semantics (and a slightly smaller memory footprint).
Writer
Writer is a buffer that implements the io.Writer interface and expands the buffer as needed. The reset functionality allows for better memory reuse. After calling Reset, it will overwrite the current buffer and thus reduce allocations.
Lexer
Lexer is a read buffer specifically designed for building lexers. It keeps track of two positions: a start and end position. The start position is the beginning of the current token being parsed, the end position is being moved forward until a valid token is found. Calling Shift will collapse the positions to the end and return the parsed []byte.
Moving the end position can go through Move(int) which also accepts negative integers. One can also use Pos() int to try and parse a token, and if it fails rewind with Rewind(int), passing the previously saved position.
Peek(int) byte will peek forward (relative to the end position) and return the byte at that location. PeekRune(int) (rune, int) returns UTF-8 runes and its length at the given byte position. Upon an error Peek will return 0, the user must peek at every character and not skip any, otherwise it may skip a 0 and panic on out-of-bounds indexing.
Lexeme() []byte will return the currently selected bytes, Skip() will collapse the selection. Shift() []byte is a combination of Lexeme() []byte and Skip().
When the passed io.Reader returned an error, Err() error will return that error even if not at the end of the buffer.
StreamLexer
StreamLexer behaves like Lexer but uses a buffer pool to read in chunks from io.Reader, retaining old buffers in memory that are still in use, and re-using old buffers otherwise. Calling Free(n int) frees up n bytes from the internal buffer(s). It holds an array of buffers to accommodate for keeping everything in-memory. Calling ShiftLen() int returns the number of bytes that have been shifted since the previous call to ShiftLen, which can be used to specify how many bytes need to be freed up from the buffer. If you don't need to keep returned byte slices around, call Free(ShiftLen()) after every Shift call.
Strconv
This package contains string conversion function much like the standard library's strconv package, but it is specifically tailored for the performance needs within the minify package.
For example, the floating-point to string conversion function is approximately twice as fast as the standard library, but it is not as precise.
CSS
This package is a CSS3 lexer and parser. Both follow the specification at CSS Syntax Module Level 3. The lexer takes an io.Reader and converts it into tokens until the EOF. The parser returns a parse tree of the full io.Reader input stream, but the low-level Next function can be used for stream parsing to returns grammar units until the EOF.
HTML
This package is an HTML5 lexer. It follows the specification at The HTML syntax. The lexer takes an io.Reader and converts it into tokens until the EOF.
JS
This package is a JS lexer (ECMA-262, edition 6.0). It follows the specification at ECMAScript Language Specification. The lexer takes an io.Reader and converts it into tokens until the EOF.
JSON
This package is a JSON parser (ECMA-404). It follows the specification at JSON. The parser takes an io.Reader and converts it into tokens until the EOF.
SVG
This package contains common hashes for SVG1.1 tags and attributes.
XML
This package is an XML1.0 lexer. It follows the specification at Extensible Markup Language (XML) 1.0 (Fifth Edition). The lexer takes an io.Reader and converts it into tokens until the EOF.
License
Released under the MIT license.