Every checker in this group works on markup you paste, not on a URL it fetches. That is a deliberate limit: the page never leaves your browser, so the checks work on staging builds, pages behind a login and drafts that are not live yet. The trade-off is that you choose which HTML to inspect. Copy from view-source for what the server sends, which is what a crawler sees first, or from the Elements panel of your developer tools for what scripts produced after loading.
For structure and accessibility, the heading structure checker draws the H1 to H6 tree and flags skipped levels; a jump from H2 to H4 matters to screen-reader users, who navigate a page by its headings. The image alt checker lists every image with its alt text and judges it. A decorative image should carry an empty alt="" rather than no attribute at all, and an informative one needs text that says what the image shows, not "image of". The deprecated HTML tags checker finds obsolete elements such as font and center and the presentational attributes that belong in CSS.
For links and security, the link attributes checker audits rel, target and anchor text across every link — nofollow, sponsored and ugc for paid or user-generated links, noopener where a link opens a new tab. The mixed content checker finds http:// resources on an HTTPS page; browsers block insecure scripts outright, so one old embed can break both the padlock and the feature it powered. The Google Analytics checker finds GA4, Tag Manager and retired Universal Analytics snippets, including the same measurement ID configured twice, which doubles every page view.
Two more round off a review. The media query checker lists the breakpoints in your CSS and finds overlaps and gaps: with max-width: 767px and min-width: 768px, a zoomed viewport of 767.5 pixels matches neither query. The SEO word counter checks how often a focus keyphrase appears in a draft, its density, and whether it turns up in the first paragraph. Use it to confirm the page names its subject early, not to hit a percentage: there is no ideal density, and a phrase repeated to reach one reads badly to people before it reads badly to Google.