How can you determine whether important content is available in rendered HTML?

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with

rendered DOM

and then validate with Search Console URL Inspection.

If the important content exists only after JavaScript, that is not automatically a problem. The question is whether Google can reliably render and process it.

Also test representative templates rather than only one page.

For an e-commerce site, test:

• product;
• category;
• filtered category;
• search;
• out-of-stock product;
• discontinued product.

Google specifically recommends URL Inspection for checking how Google sees rendered pages.