Add comments to tests as requested by @wjdp

Author: tomtom5152

htmltest/check-link.go

@@ -357,6 +357,7 @@ func (hT *HTMLTest) checkMailto(ref *htmldoc.Reference) {
 
 	domain := strings.Split(ref.URL.Opaque, "@")[1]
 
+	// loop over the current domain until we have a valid result or have exhausted all possibilities
 	for domain != "" {
 		// if a simple MX lookup works, we are done, continue
 		if _, err := net.LookupMX(domain); err == nil {

htmltest/check-link_test.go

@@ -391,27 +391,32 @@ func TestMailtoInvalidFormat(t *testing.T) {
 }
 
 func TestMailtoInvalidCname(t *testing.T) {
-	// fails for invalid mailto links
+	// rubdomain with just a CNAME record without MX pointers
+	// email is still deliverable so should be accepted
+	// the records have been carefully created to ensure this is valid and will later fall under one of the other
+	// pass conditions
 	hT := tTestFile("fixtures/links/invalid_mailto_cname.html")
 	tExpectIssueCount(t, hT, 0)
 }
 
 func TestMailtoInvalidCnameLoop(t *testing.T) {
-	// fails for invalid mailto links
+	// subdomain with just a CNAME record without MX records in a loop
+	// should be detected and fail as we can never resolve this
 	hT := tTestFile("fixtures/links/invalid_mailto_cname_loop.html")
 	tExpectIssueCount(t, hT, 1)
 	tExpectIssue(t, hT, "email domain could not be resolved correctly", 1)
 }
 
 func TestMailtoInvalidNoRecords(t *testing.T) {
-	// fails for invalid mailto links
+	// domain with no records associated at all, must fail as completely unroutable
 	hT := tTestFile("fixtures/links/invalid_mailto_norecords.html")
 	tExpectIssueCount(t, hT, 1)
 	tExpectIssue(t, hT, "email domain could not be resolved correctly", 1)
 }
 
 func TestMailtoInvalidAFallback(t *testing.T) {
-	// fails for invalid mailto links
+	// domain without MX records but with a valid A record
+	// email can be routed to the A record so should be accepted
 	hT := tTestFile("fixtures/links/invalid_mailto_fallback.html")
 	tExpectIssueCount(t, hT, 0)
 }