What a non-JS crawler actually sees
This fetches your page with a Googlebot-style user agent and no JavaScript execution, then extracts exactly what that kind of basic crawl would see: the title, meta description, heading structure in document order, link and image counts, and the visible text content. If your page relies heavily on client-side JavaScript to render its main content, this view can look strikingly empty compared to what a human visitor sees in a browser.
Why this isn't the complete indexing picture
Modern Googlebot does render JavaScript in a second rendering pass, so this is not a complete picture of how Google ultimately indexes a JS-heavy page - but it accurately shows what is available immediately, before any rendering happens, which matters for other crawlers and for how quickly your core content gets discovered.
What the heading structure reveals
Headings are shown in document order with their nesting level, which makes it easy to spot a genuinely common SEO mistake: skipping heading levels (jumping straight from H1 to H4, for instance), which can muddy the topical outline search engines use to understand what a page is actually about. Using more than one H1 on a page is a different matter - Google's John Mueller has said repeatedly that multiple H1s don't hurt rankings, since Google's systems handle that pattern fine - but a single, clear H1 is still good practice for accessibility and for readers skimming a page's outline.
Why missing alt text gets its own count
Images without alt text are both an accessibility gap (screen readers have nothing to announce for that image) and a missed opportunity for image search visibility, since alt text is one of the signals search engines use to understand what an image depicts. A high missing-alt count across a page is worth fixing on both fronts.