Contrast between placeholder and code-input background often inaccessible

[WebCoder49]

Good idea to fix this, likely dynamically with JavaScript (CSS contrast-color has only experimental WebKit support and no other browser support for now).

luminance boundaries between colours: #000000 | #333333 | #757575 | #bbbbbb | #ffffff
placeholder colours                 :     #ffffff   #aaaaaa   #444444   #00000

[WebCoder49]

Prevent this overriding custom styling / #182

[WebCoder49]

Should this (autosetting colour for contrast, with desired minimum contrast input) be...

In the core file	In a custom plugin
☺Can auto-ship to existing usage (ALTHOUGH many dependents still rely on manual updates)	☺No need for excess code transfer when contrast is already carefully checked (ALTHOUGH kept short; major version 2 already has quite a lot of excess code which should be slimmed down in major version 3)
☺Encourages accessibility by default	☺No need for excess performance impact when contrast is already carefully checked (ALTHOUGH performance impact should be minimal, polling for colours but only calculating when they change - TODO test)
☺Easier to implement as plugin would require adding new parts to its exposed API (although such an extension may be useful for specific cases of future development)

What do you think, everyone?

[WebCoder49]

For minimum performance cost and more flexibility, using colour of highlighted block instead of calculated contrast colour - the highlighter theme and browser are responsible for colourful styling including contrast, and seem to work well with this.

[WebCoder49]

Fixed with #195