What does fixation strength change?
The proportion of each word that gets bolded. Level 1 bolds the first 35% of a word's letters, level 5 bolds 75%, with levels 2 to 4 in between. Short words of three letters or fewer are handled separately and bold roughly 40%, always at least one letter, so that 'the' does not become entirely bold.
Does bionic reading actually make you read faster?
Probably not, on the available evidence. Independent studies attempting to replicate the claimed speed benefit have generally found no significant improvement, and a few found a small slowdown. What some readers, particularly some with ADHD, report is that it helps them keep their place and resist skipping lines, which is a genuine subjective benefit even though it is not a speed gain.
Can I use the output on my own site?
Yes. It is plain HTML with <b> tags and <br /> for line breaks, with all your text HTML-escaped, so it is safe to insert into a page. Offer it as a toggle rather than a default, because a reader who does not want it will find a whole page of half-bold text harder to read, not easier.
Does it help screen reader users?
No. Screen readers announce text, not typography, so the <b> tags are ignored entirely. Bionic reading is a purely visual technique for sighted readers. It is not an accessibility feature in the assistive-technology sense, and describing it as one is misleading.
Why does my pasted HTML show up as visible tags?
Because the input is treated as plain text and escaped before processing, so a < becomes < in the output. That is deliberate: it means the converter cannot be used to inject markup, and it means you should paste the text content rather than the markup of whatever you want to convert.