[ZYbCRq22HbJ2y7]

for all the lowercase complainers:

   document.querySelectorAll("p").forEach(p => p.textContent = p.textContent.replace(/(^\w)|([.?!]\s*\w)|( i'? )/g, m => m.toUpperCase()));

[micku7zu]

Or using CSS p:first-letter{ text-transform: capitalize;}:

  document.styleSheets[0].insertRule("p:first-letter{ text-transform: capitalize;}",document.styleSheets[0].cssRules.length);

[mkl]

That only works for paragraphs, not sentences.