What this tool checks
Five categories of on-page SEO signal, all extracted directly from the page's HTML: the <title> tag, the meta description, the canonical link, Open Graph tags (used by Facebook, LinkedIn, Slack, and most link-preview systems), and Twitter Card tags. Title and description are also checked against recommended length ranges — not because there's a hard rule enforced anywhere, but because search engines visually truncate results beyond roughly 60 and 160 characters respectively.
Why these specific tags
Title and meta description are what a searcher actually reads in results before clicking — they're your pitch, not just metadata. The canonical link prevents duplicate-content confusion when the same page is reachable through multiple URLs. Open Graph and Twitter Card tags control how a link looks when shared on social platforms or messaging apps — without them, a shared link falls back to whatever a platform guesses from the page, which is often an unrelated image or no image at all.
What "good" length means here
These are guidelines, not hard search-engine rules — a page can rank perfectly well with a title outside this range. But titles under 10 characters rarely carry enough context to be useful, and both title and description get visually truncated by search engines past the upper bound, which is the more common real-world problem this check catches.