Isn't a "site returns 200 OK" check enough?

No. A page can return 200 and still show a database error, a blank screen, or a maintenance notice — and an unexpected redirect to a login or parked page can mislead a check just as badly. Verify the status, the final URL after redirects, and a stable piece of text unique to the correct page; where supported, also fail the check when a known error message appears.

Why monitor certificate and domain expiry separately?

Because they fail on a date, not from load — and the automation guarding them can break quietly. Certificate renewal usually runs automatically, but DNS, account or validation problems can stop it, so alert at several thresholds while there's time to repair the renewal. For the domain, track the registrar's actual billing deadline as well as the registry expiry date — they aren't always the same thing — and treat monitoring as a safety net around auto-renew, not a replacement for it.

What should I monitor beyond the homepage?

The function that makes you money or matters most, tested safely: a clearly tagged test message through the contact form to a controlled mailbox, a restricted test account for login, the payment provider's test mode for checkout — never real orders or charges. And where you control the hosting, watch the internal signs too: error rates, disk space, database health, scheduled jobs.

Would you know before your customers if something broke?

The worst way to learn your website is down is from a customer who couldn't reach it — or worse, one who tried to buy something and couldn't. Monitoring flips that around: you're alerted quickly, ideally before a customer reports it. Two kinds work together: external checks detect failures after they begin; internal health monitoring can flag rising error rates or a filling disk before the site goes down.

What a good external check looks like

Verify the response, not the connection. Check the expected status code, the final URL after redirects, the response time — and a stable piece of content unique to the correct page (not a navigation label that also appears on the error template). Where supported, also fail on known error text. A blank page returns 200 OK; so does a parked domain.
Pick the interval by business impact. How long may an outage go unnoticed? A shop or booking system may justify every-minute checks; every five minutes is fine for a small informational site. Heavier browser checks can run less often than the lightweight ping.
Multiple locations, sensible alarm policy. Checking from several places only prevents false alarms if the alert requires confirmation — repeated failures, or more than one location. Read the pattern: one location failing is possibly routing or the probe itself; several is likely a real outage; one region failing repeatedly is real too, just not global. Record the isolated failures instead of paging on them.

Test a real function, safely

A plain HTTP check is cheap — run it often. A browser-based (synthetic) check loads the page like a visitor, with scripts and styles, and can walk through the step that matters: does the contact form actually send, does login accept a session, does checkout start? Run it safely: a clearly tagged test message to a controlled mailbox, a restricted test account, the payment provider's test mode — never real orders, charges or messages to real people. Store the test credentials securely and make sure the monitoring service doesn't expose passwords, personal data or form contents in its logs, screenshots or alerts. And know the limit: a sandbox transaction proves the sandbox — pair synthetic checks with production error monitoring where you can.

The calendar failures — and the automation behind them

TLS certificates usually renew automatically — until a DNS, account or validation problem quietly stops the automation. Alert at several thresholds (not just the final day), and check the certificate matches the hostname.
Domain renewal: monitor the registration status, but track the registrar's real billing deadline and payment method too — the registry's expiry date and the registrar's deletion date aren't always the same, especially for country-code domains. Monitoring is the safety net around auto-renew and current contact details, not a substitute.

Watch the warning signs inside

Where you control the hosting or application, monitor error rates, disk space, database connectivity and scheduled jobs — external checks say visitors have a problem; internal ones say why, and sometimes say it early enough to prevent the outage. On managed hosting the provider may own these checks; then at least know who receives those alerts.

Alerts that reach a human

An alert nobody reads is not monitoring — and neither is an alert everybody ignores.

Route alerts through more than one dependable channel (push plus SMS beats three emails to the same inbox), to a named person with an escalation path: who's first, who's next, how they reach the host or DNS provider.
Fight alert fatigue: require confirmation before declaring an outage, suppress alerts during planned maintenance, and send a recovery notice when service returns.
Test the alert path itself periodically — trigger a controlled failure and confirm the right people get it — and review the monitors whenever URLs, forms or providers change. A monitor aimed at last year's page stays green while this year's checkout is broken.

Takeaway: Monitoring is not one green homepage check. It's the proven chain from detecting the right failure, through a reliable alert, to a person who knows what to do next.

What to do

Set up a frequent external check of the canonical HTTPS URL, verifying status, final URL and stable real content.
Check from several locations, with alerts requiring repeated or multi-location confirmation.
Add a browser-based check of your most important function — tagged test data, test accounts, test mode; never real transactions.
Monitor TLS validity at several thresholds, and domain status alongside the registrar's actual billing deadline.
Watch internal health where you can — errors, disk, database, scheduled jobs — or know who does.
Send alerts through two channels to a named owner with an escalation path; suppress maintenance, notify recovery, and test the alert route itself.