chore(docs): improve visual appearance of images in README.md

It is more readable to call `startsWith` than to call `indexOf` and
compare the result with zero to determine whether a string starts with a
given prefix.

`startsWith` is also more efficient as it only compares at the beginning
of the string, while `indexOf` searches the entire string.

Signed-off-by: Eng Zer Jun <engzerjun@gmail.com>

…startsWith

Replace `indexOf === 0` with `startsWith`

This allows subclasses to call and override this method. This PR also
adds a shared type declaration for the common argument type used to
add or create new nodes, which eventually gets passed into
`normalize`.

This is useful to call from subtypes.

Fix a few TypeDoc warnings

Add protected methods

This makes it possible for callers to pass in their own
`ReadonlyArray`s, and implicitly documents that these arguments aren't
modified.

Use `ReadonlyArray` for argument types that aren't modified

* Pretty CssSyntaxError from oneline (minified) css

* - cut minified css in place
- extend minified css length to 160

* - cut minified css considering block