The creator is taking advantage of the fact that most of us read words semi-holistically (if not, it's a hell of a coincidence). Research has demonstrated that as long as the first and last letter remain unchanged, we can read passably well. (Though "bookmarklet" took me some time to decipher.) Difficulty does varies from anagram to anagram, however.

I guess the idea is to mimic the increased cognitive load of dyslexia to give non-dyslexic people a feel for it, without making each word as difficult as deciphering full anagrams.

This is a bit of a myth, but it also has some truth to it:

https://en.wikipedia.org/wiki/Typoglycemia

Interesting. For me (not dyslexic) this demonstration hurts my head so badly that I can't read more than one sentence at a time without looking away.

Some of the words pop out to me immediately, but others I have to stare at for a while before I can figure out what the word is. The way the letters jump around puts an immediate stress on my brain that is really unpleasant.

I have to agree with you, I am dyslexic and found this not much more difficult to read than normal text. I find when reading I will stare at words in sentences knowing what they say and what they mean but not knowing or being able to understand the meaning of the sentence overall. I will often re-read the sentence over and over and end up moving on not understanding what it means, just knowing certain words were mentioned.

Especially when the first and last letters aren't changing as in this example.

It looks like it's just doing random anagrams of all the letters in the word, except the first and last.

From what I know of dyslexia, wouldn't it be more appropriate to replace each letter with a weighted random selection from groupings of similar glyphs?

  a e g 6 9 @
  b d p q P
  c n u C G U ( )
  f t k x H K X +
  h y F T Y
  i ! : ;
  j r J L 7 [ ]
  l I 1 / | \
  m w E 3 { }
  o D O Q 0
  s z S Z 2 5 ? $
  v A V ^ 4 < >
  B R 8 &
  M N W
  . , ' `
  * " ~
  # =
  %
  _ -

There are also Unicode glyphs that resemble English letters turned upside down or sideways.

Some psychology researchers can really mess with your brain by aiming a camera at your eye and changing the letters of a text during a saccade. When your eyes are moving to a new fixation point, you become temporarily blind to changes in what you are looking at for a fraction of a second.

Obviously, that can't be done in a webpage demonstration without some fool giving control of their camera to your javascript. Then you would have to calibrate the gaze detection. And your program would have to run fast enough to update during a saccade.

It seems like only replacing similar glyphs would make it even easier to read.

Yes, but now try passing a spelling test in elementary school when "word" looks like "worb"

Of course. I'm not trying to downplay the challenge of dyslexia. Quite the opposite, in fact. I'm expressing doubt that this demonstration at all depicts the struggle of reading with dyslexia.

Yeah, the first letter is a biggie. I realized that after I left my last comment. I bet if the first letter was able to transpose, things would get way more difficult.

I found it readable, but I am glad I didn't have to learn reading like that. Maybe then I would never have reached a level that allows me to read that.