Speed, uptime, and a page with a point.
Three things decide whether the research survives contact with a real website.
Speed. The HTML should arrive quickly. Type should stay still while it loads. A phone on an ordinary connection should get the headline without waiting on unused scripts. Images are sized for the slot they sit in. We ship a light page, and we judge it by opening it.
Reliability. Rankings attach to a URL. A site that falls over, or a plugin update that blanks the page, hands that query to someone else. Client sites run on a CMS you can edit, with redirects when an address changes, and a preview so a draft is separate from the live page.
Search readability. The title tag is the line in the results. The H1 is the promise on the page, and it is text. The first paragraph answers the query. Internal links name the next page in words a person would use. The sitemap lists the pages that should be found.
Why HTML-first: crawlers and visitors read the same words. Heavy themes and duplicate URLs work against that. We use a CMS we maintain so the page stays lean—we describe the outcome, not our internal server recipes.