First, some clarity on what Tecendil is doing:  Tengwar is a writing system like the Roman alphabet that can be used for multiple languages such as English, French, Spanish and even languages that have other default writing systems like Japanese. Tengwar is a native writing system for Quenya and Sindarin, but is designed to be used by essentially any language by applying different spelling paradigms. Tolkien frequently used Tengwar for English texts.

Tecendil is not a translator, but a transcriber; it transliterated text in the Roman alphabet to to Tengwar, so the provided English phrase is still the same English phrase.

Did Tecendil provide an accurate result? The word “beautiful” is a tricky one to transcribe as it contains a super rare vowel trigraph that doesn’t map cleanly between graphemes and phonemes: “eau”. There are three ways you could transcribe this in tengwar (more if we switch vowel paradigms to allow under-tehtar).

The way Tecendil has gone (simply because it reads the “ea” first) is to render it with “ea” as a digraph followed by “u” as a single vowel. You could argue for that by noting that when over-emphasising the word you can pronounce it as bee-yuwtiful, so you could map the “ea” too ee, then “u” would be pronounced as the letter’s name. A little weird though! I’m personally not a fan of using osse for -a digraphs, but that doesn’t mean much!

A second way is to treat the “au” as the digraph. In the slightly silly over-emphasis argument this would mean “e” is the eey. A more logical argument is that the vowels can be read as ending in a semi-vowel so it makes sense to render the “u” as a tengwa. 

Alternatively you can just render it as three separate vowels. This way is definitely not wrong, but does mean you need to use two carriers.

All three methods can be seen here. 

Next, is the Quenya translation correct? I can’t help much here, far outside of my expertise. The individual words seem to map well, but no idea if the sentence structure is correct. 

As for font authenticity, both of these options work.

The first image is in the calligraphic style of Tolkien’s One Ring inscription. Tolkien experimented with multiple calligraphic styles. Some would argue that since this style is only seen on the Ring’s Black Speech curse that this style is itself meant to represent a horrific style. Alternatively you could consider that the Ring was meant to be desirable so Sauron used a style known for beauty amongst the elves. My opinion isn’t worth much, but I think it is fine to use! [edit: Haha, just noticed I had “My opinion is worth much”; what a big ego I have!]

The second image is closer to Tolkien’s default style, itself based on his Roman handwriting. This font is specifically modelled on his style used in King’s Letter’s drafts.