diff --git a/Userland/Libraries/LibC/arpa/inet.h b/Userland/Libraries/LibC/arpa/inet.h
index b5c4d64a0d7..fcfbc3ec3f6 100644
--- a/Userland/Libraries/LibC/arpa/inet.h
+++ b/Userland/Libraries/LibC/arpa/inet.h
@@ -6,8 +6,11 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/arpa_inet.h.html
 #include <inttypes.h>
 #include <netinet/in.h>
+
 #include <sys/cdefs.h>
 #include <sys/socket.h>
 
diff --git a/Userland/Libraries/LibC/fcntl.h b/Userland/Libraries/LibC/fcntl.h
index 73fa5ec9c9c..ceb5896c474 100644
--- a/Userland/Libraries/LibC/fcntl.h
+++ b/Userland/Libraries/LibC/fcntl.h
@@ -7,6 +7,11 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/fcntl.h.html
+#include <sys/stat.h>
+#include <unistd.h>
+
 #include <Kernel/API/POSIX/fcntl.h>
 #include <Kernel/API/POSIX/sys/stat.h>
 #include <sys/cdefs.h>
diff --git a/Userland/Libraries/LibC/langinfo.h b/Userland/Libraries/LibC/langinfo.h
index 1545e18c2dd..f5836915ba6 100644
--- a/Userland/Libraries/LibC/langinfo.h
+++ b/Userland/Libraries/LibC/langinfo.h
@@ -6,7 +6,10 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/langinfo.h.html
 #include <nl_types.h>
+
 #include <sys/cdefs.h>
 
 __BEGIN_DECLS
diff --git a/Userland/Libraries/LibC/netdb.h b/Userland/Libraries/LibC/netdb.h
index b825f631821..f0a7d28e759 100644
--- a/Userland/Libraries/LibC/netdb.h
+++ b/Userland/Libraries/LibC/netdb.h
@@ -6,6 +6,12 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/netdb.h.html
+#include <inttypes.h>
+#include <netinet/in.h>
+#include <sys/socket.h>
+
 #include <sys/cdefs.h>
 #include <sys/types.h>
 
diff --git a/Userland/Libraries/LibC/netinet/in.h b/Userland/Libraries/LibC/netinet/in.h
index 6a3163d2e67..074c0cda515 100644
--- a/Userland/Libraries/LibC/netinet/in.h
+++ b/Userland/Libraries/LibC/netinet/in.h
@@ -6,6 +6,11 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/netinet_in.h.html
+#include <inttypes.h>
+#include <sys/socket.h>
+
 #include <Kernel/API/POSIX/netinet/in.h>
 #include <endian.h>
 #include <sys/cdefs.h>
diff --git a/Userland/Libraries/LibC/pthread.h b/Userland/Libraries/LibC/pthread.h
index f5f76b6dfdc..7129fbdde43 100644
--- a/Userland/Libraries/LibC/pthread.h
+++ b/Userland/Libraries/LibC/pthread.h
@@ -6,12 +6,15 @@
 
 #pragma once
 
-#include <bits/pthread_integration.h>
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/pthread.h.html
 #include <sched.h>
+#include <time.h>
+
+#include <bits/pthread_integration.h>
 #include <stdint.h>
 #include <sys/cdefs.h>
 #include <sys/types.h>
-#include <time.h>
 
 __BEGIN_DECLS
 
diff --git a/Userland/Libraries/LibC/sched.h b/Userland/Libraries/LibC/sched.h
index 61d6fd77c7d..b6867df04be 100644
--- a/Userland/Libraries/LibC/sched.h
+++ b/Userland/Libraries/LibC/sched.h
@@ -6,6 +6,10 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sched.h.html
+#include <time.h>
+
 #include <Kernel/API/POSIX/sched.h>
 #include <sys/cdefs.h>
 #include <sys/types.h>
diff --git a/Userland/Libraries/LibC/semaphore.h b/Userland/Libraries/LibC/semaphore.h
index ed009f0c7ed..7a415246bfb 100644
--- a/Userland/Libraries/LibC/semaphore.h
+++ b/Userland/Libraries/LibC/semaphore.h
@@ -6,6 +6,11 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/semaphore.h.html
+#include <fcntl.h>
+#include <time.h>
+
 #include <limits.h>
 #include <pthread.h>
 #include <sys/cdefs.h>
diff --git a/Userland/Libraries/LibC/signal.h b/Userland/Libraries/LibC/signal.h
index 4aa00487333..c3cb70a3864 100644
--- a/Userland/Libraries/LibC/signal.h
+++ b/Userland/Libraries/LibC/signal.h
@@ -6,6 +6,10 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/signal.h.html
+#include <time.h>
+
 #include <Kernel/API/POSIX/signal.h>
 #include <Kernel/API/POSIX/ucontext.h>
 #include <bits/sighow.h>
diff --git a/Userland/Libraries/LibC/spawn.h b/Userland/Libraries/LibC/spawn.h
index 8531e670445..9458628da33 100644
--- a/Userland/Libraries/LibC/spawn.h
+++ b/Userland/Libraries/LibC/spawn.h
@@ -13,8 +13,11 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/spawn.h.html
 #include <sched.h>
 #include <signal.h>
+
 #include <sys/cdefs.h>
 #include <sys/types.h>
 
diff --git a/Userland/Libraries/LibC/stdio.h b/Userland/Libraries/LibC/stdio.h
index 83c349f57d2..97c58799dd1 100644
--- a/Userland/Libraries/LibC/stdio.h
+++ b/Userland/Libraries/LibC/stdio.h
@@ -8,6 +8,10 @@
 
 #define _STDIO_H // Make GMP believe we exist.
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/stdio.h.html
+#include <stddef.h>
+
 #include <Kernel/API/POSIX/stdio.h>
 #include <bits/FILE.h>
 #include <limits.h>
diff --git a/Userland/Libraries/LibC/stdlib.h b/Userland/Libraries/LibC/stdlib.h
index fbe0b7ca55b..8b1f937d353 100644
--- a/Userland/Libraries/LibC/stdlib.h
+++ b/Userland/Libraries/LibC/stdlib.h
@@ -6,8 +6,14 @@
 
 #pragma once
 
-#include <bits/wchar.h>
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/stdlib.h.html
+#include <limits.h>
+#include <math.h>
 #include <stddef.h>
+#include <sys/wait.h>
+
+#include <bits/wchar.h>
 #include <sys/cdefs.h>
 #include <sys/types.h>
 
diff --git a/Userland/Libraries/LibC/string.h b/Userland/Libraries/LibC/string.h
index 23ec5ef69f9..0d96455205e 100644
--- a/Userland/Libraries/LibC/string.h
+++ b/Userland/Libraries/LibC/string.h
@@ -7,6 +7,10 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/string.h.html
+#include <stddef.h>
+
 #include <sys/cdefs.h>
 #include <sys/types.h>
 
diff --git a/Userland/Libraries/LibC/sys/resource.h b/Userland/Libraries/LibC/sys/resource.h
index 7d3f265396d..ce954d9e399 100644
--- a/Userland/Libraries/LibC/sys/resource.h
+++ b/Userland/Libraries/LibC/sys/resource.h
@@ -6,9 +6,12 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_resource.h.html
+#include <sys/time.h>
+
 #include <stdint.h>
 #include <sys/cdefs.h>
-#include <sys/time.h>
 
 __BEGIN_DECLS
 
diff --git a/Userland/Libraries/LibC/sys/select.h b/Userland/Libraries/LibC/sys/select.h
index 02e9f0d6f67..f16307c56c4 100644
--- a/Userland/Libraries/LibC/sys/select.h
+++ b/Userland/Libraries/LibC/sys/select.h
@@ -6,8 +6,12 @@
 
 #pragma once
 
-#include <fd_set.h>
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_select.h.html
 #include <signal.h>
+#include <time.h>
+
+#include <fd_set.h>
 #include <string.h>
 #include <sys/cdefs.h>
 #include <sys/time.h>
diff --git a/Userland/Libraries/LibC/sys/socket.h b/Userland/Libraries/LibC/sys/socket.h
index f505992c022..bb8aee03edc 100644
--- a/Userland/Libraries/LibC/sys/socket.h
+++ b/Userland/Libraries/LibC/sys/socket.h
@@ -6,6 +6,10 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_socket.h.html
+#include <sys/uio.h>
+
 #include <Kernel/API/POSIX/sys/socket.h>
 #include <sys/cdefs.h>
 #include <sys/un.h>
diff --git a/Userland/Libraries/LibC/sys/time.h b/Userland/Libraries/LibC/sys/time.h
index 00d741f6e9c..7dfa1891049 100644
--- a/Userland/Libraries/LibC/sys/time.h
+++ b/Userland/Libraries/LibC/sys/time.h
@@ -6,6 +6,10 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_time.h.html
+#include <sys/select.h>
+
 #include <Kernel/API/POSIX/sys/time.h>
 #include <sys/cdefs.h>
 #include <time.h>
diff --git a/Userland/Libraries/LibC/sys/wait.h b/Userland/Libraries/LibC/sys/wait.h
index a5292f9ffe0..5d1fd7af36c 100644
--- a/Userland/Libraries/LibC/sys/wait.h
+++ b/Userland/Libraries/LibC/sys/wait.h
@@ -6,6 +6,10 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_wait.h.html
+#include <signal.h>
+
 #include <Kernel/API/POSIX/signal.h>
 #include <Kernel/API/POSIX/sys/wait.h>
 #include <sys/cdefs.h>
diff --git a/Userland/Libraries/LibC/time.h b/Userland/Libraries/LibC/time.h
index 538c5e025d4..66a09d32bd6 100644
--- a/Userland/Libraries/LibC/time.h
+++ b/Userland/Libraries/LibC/time.h
@@ -6,6 +6,10 @@
 
 #pragma once
 
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/time.h.html
+#include <signal.h>
+
 #include <Kernel/API/POSIX/time.h>
 #include <sys/cdefs.h>
 
diff --git a/Userland/Libraries/LibC/wchar.h b/Userland/Libraries/LibC/wchar.h
index b6052f7c81d..090f340c436 100644
--- a/Userland/Libraries/LibC/wchar.h
+++ b/Userland/Libraries/LibC/wchar.h
@@ -6,10 +6,18 @@
 
 #pragma once
 
-#include <bits/FILE.h>
-#include <bits/wchar_size.h>
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/wchar.h.html
+#include <ctype.h>
 #include <stdarg.h>
 #include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+
+#include <bits/FILE.h>
+#include <bits/wchar_size.h>
 #include <sys/cdefs.h>
 
 __BEGIN_DECLS
diff --git a/Userland/Libraries/LibC/wctype.h b/Userland/Libraries/LibC/wctype.h
index 514371c1638..a154f983b80 100644
--- a/Userland/Libraries/LibC/wctype.h
+++ b/Userland/Libraries/LibC/wctype.h
@@ -6,11 +6,20 @@
 
 #pragma once
 
-#include <assert.h>
+// Includes essentially mandated by POSIX:
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/wctype.h.html
 #include <ctype.h>
-#include <sys/cdefs.h>
+#include <stdarg.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
 #include <wchar.h>
 
+#include <assert.h>
+#include <sys/cdefs.h>
+
 __BEGIN_DECLS
 
 typedef long wctrans_t;