diff --git a/main.go b/main.go
index e20dd12..635a5ad 100644
--- a/main.go
+++ b/main.go
@@ -8,6 +8,8 @@ import (
 	"strings"
 	"time"
 	"strconv"
+	"path"
+	"mime"
 )
 
 type BookInfo struct {
@@ -83,7 +85,7 @@ func validate(archive *zip.Reader) bool {
 			fmt.Println("WARNING: No index provided, checking default index.gmi")
 			return isValidContent("index.gmi")
 		} else {
-			return isValidContent(bookInfo.Index)
+			return isValidContent(path.Join(bookInfo.Index))
 		}
 	} else {
 		fmt.Println("WARNING: This is not a book but a capsule archive")
@@ -154,6 +156,7 @@ func isValidMetadata(file *zip.File) bool {
 				_, err := strconv.ParseUint(value, 10, 32)
 				if err != nil {
 					fmt.Println("FATAL: Bad format in", key, err)
+					return false
 				}
 				bookInfo.Published = value
 			case "publishDate":
@@ -217,7 +220,53 @@ func isValidContent(filePath string) bool {
 		return isValidContent(filePath + "index.gmi")
 	}
 
-	//todo
+	reader, err := file.Open()
+	if err != nil {
+		fmt.Println("FATAL:", file.Name, "can't be opened")
+		return false
+	}
+	defer reader.Close()
+
+	scanner := bufio.NewScanner(reader)
+	for scanner.Scan() {
+		// we only check links
+		if strings.HasPrefix(scanner.Text(), "=>") && len(scanner.Text()) > 2 {
+			link := strings.Trim(scanner.Text()[2:], " ")
+			sepPosition := strings.Index(link, " ")
+			var location, text string
+			if sepPosition == -1 {
+				location = link
+				text = ""
+			} else {
+				location = strings.Trim(link[:sepPosition], " ")
+				text = strings.Trim(link[sepPosition+1:], " ")
+			}
+
+			// transform location to have absolute gpub path
+			if strings.HasPrefix(location, "/") { // if begin with / we begin at root but without leading / and simplified (.. and .)
+				location = path.Join(location[1:])
+			} else { // else join this file dir and link location
+				location = path.Join(path.Dir(filePath), location)
+			}
+
+			if isImage(location) {
+				if text == "" {
+					fmt.Println("FATAL: image without alternative text in", file.Name, scanner.Text())
+					return false
+				}
+			}
+
+			if isGemtext(location) {
+				if !isValidContent(location) {
+					return false
+				}
+			}
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		fmt.Println("FATAL: error while scanning", file.Name, err)
+		return false
+	}
 
 	return true
 }
@@ -225,3 +274,13 @@ func isValidContent(filePath string) bool {
 func isVersionValid() bool {
 	return bookInfo.GpubVersion == "1.0.0"
 }
+
+func isImage(file string) bool {
+	ext := file[strings.LastIndex(file, "."):]
+	return strings.HasPrefix(mime.TypeByExtension(ext), "image")
+}
+
+func isGemtext(file string) bool {
+	ext := file[strings.LastIndex(file, "."):]
+	return ext == ".gmi" || ext == ".gemini"
+}