You really need two things. First, a font that can display the characters, e.g. one of the Free Tengwar Font Project fonts (disclaimer: I am one of the initiators). And then, instructions that tell your viewer to use the font.
It would be most convenient if these two things could be provided server side. This should be possible (it is what Tecendil does), but we would have to convince the good folks from sdf.org to do it for us.
When using an internet browser, both the font and the instructions could be included in a custom user style script to be installed with a user style browser add-on.
When you are using an app, the app might be tweaked to provide the font. I doubt that there are apps where you can select a custom font, so this would probably mean some app development.
Otherwise, you can always paste the text into the input field of https://tecendil.com where it will be displayed correctly (the output field might garble it up, though).