FirstImpression.io is an advertising technology; this list is derived from the open Wappalyzer rule set.
About 149 websites in the PageSourceSearch index contain the code that reveals it; the 50 highest-ranked are listed below.
How is FirstImpression.io detected?
Pages of sites using FirstImpression.io contain the bytes .firstimpression.io in their HTML or their own scripts, the trace the rule set detects it by. The discovery keyword is the exact bytes PageSourceSearch looks for in every stored page and script:
A literal is matched byte for byte, case-sensitively. Combine it with another keyword, a domain, a kind or a date filter on the search page to narrow the list. About FirstImpression.io
Top 50 websites using FirstImpression.io
Ranked by the domain's position in the crawl's ranked list (Common Crawl harmonic centrality), highest first. view source opens the stored page or script with the match highlighted. Computed 2026-09-28 15:45:06 UTC.
How does PageSourceSearch know a website uses FirstImpression.io?
By the source itself: Pages of sites using FirstImpression.io contain the bytes .firstimpression.io in their HTML or their own scripts, the trace the rule set detects it by. PageSourceSearch stores the raw HTML and first-party JavaScript of each crawled site and matches the keyword byte for byte, so every listed site's stored source contains it.
Is this a complete list of websites using FirstImpression.io?
No. The page lists the 50 highest-ranked sites in the index whose source holds the keyword, out of about 149 websites that do; the count is the index's estimate at the time the list was computed. Run the keyword as a search to page through all of them, or add a kind or date filter.
How do I find websites that use FirstImpression.io together with another technology?
Search for both keywords at once: several terms separated by spaces must all be found on the same site, or with the Same page toggle in the same page. For example .firstimpression.io with wp-content/plugins/ lists the WordPress sites that use FirstImpression.io.