macOS/iOS Safari and Brave browsers have "Reader mode" . Chrome has a "Reading mode" but it's more cumbersome to use because it's buried in a side menu.

For desktop browsers, I also have a bookmarklet on the bookmarks bar with the following Javascript:

  javascript: document.querySelectorAll('p, td, tr, ul, ol').forEach(elem =>  {elem.style.color = '#000'})

It doesn't darken the text on every webpage but it does work on this thread's article. (The Javascript code can probably be enhanced with more HTML heuristics to work on more webpages.)

    {elem.style.color = '#000 !important'}