Type the size, pick the colours, download the file — PNG, JPG, WebP or a few-hundred-byte SVG — or copy it as a data URI straight into your code. Generated locally, so your mockups never depend on a placeholder service being up.
Set what you want and the result appears as you go. Nothing needs uploading because there is nothing to upload — you are describing what to make, not sending us anything.
Your browser does all the work locally, using your own operating system where randomness is involved. Nothing is requested from a server, so nobody else has a copy of what you made.
Take the result to your clipboard, or save it as a file. Nothing is kept — refresh the page and it’s gone, so take what you need before you leave.