Why it matters
Browsers and crawlers need the charset within the first 1024 bytes (or in Content-Type) to decode text correctly.
How to fix it
Send Content-Type: text/html; charset=utf-8 or put <meta charset="utf-8"> first in <head> (Next.js does both).
References
More seo rules
void seo reports 60 rules in this category. What search engines need from the raw response: status codes, titles and descriptions in <head>, self-referencing canonicals, one h1, crawlable links, Open Graph, icons, robots.txt and a sitemap with real dates.
not-httpsPage served over plain HTTPorigin-redirectshttp:// or www/apex variant doesn't redirect to the canonical origin in one hophtml-lang<html lang> missing or not a valid BCP 47 tagviewportViewport meta missing, not device-width, or blocks zoomtitle-missingNo single non-empty <title> in <head>title-lengthTitle length outside 10–60 characters, or keyword-stuffedmeta-description-missingNo meta description in <head>meta-description-lengthMeta description length outside 50–160 characters