Quick update: since launching last week (www.stefan-andres.at/tools), the toolkit has grown from 3 tools to 10 same principle throughout: everything runs client-side, nothing ever touches a server.

Newest additions:

SVG Optimizer strip unnecessary code, shrink SVG file size
CSS Clamp() Calculator enter min/max size, get a ready-to-use clamp() formula for fluid typography, no media queries needed
JSON Formatter & Validator validate and format JSON, pinpoint syntax errors by line and column
TYPO3 Fluid ViewHelper Generator define your arguments, get a ready-made PHP class plus a Fluid usage example for a custom ViewHelper