Formula to check if URL in cell A1
in a Google Spreadsheet is up, using the W3C HTML Checker API
=IF(A1<>"",IF(ISERROR(FIND("non-document-error", INDEX(IMPORTDATA("https://validator.w3.org/nu/?out=json&doc=" & A1),0,2))),"✓","✗"),"")
- ✓ if the address is reachable
- ✗ if the address is not reachable
- (empty) if the cell
A1
is empty