diff --git a/cckddasd.c b/cckddasd.c
index c279da31..29c9a34c 100644
--- a/cckddasd.c
+++ b/cckddasd.c
@@ -37,6 +37,35 @@ char*         compname   [] = { "none", "zlib", "bzip2" };
 CCKD_L2ENT    empty_l2   [ CKD_NULLTRK_FMTMAX + 1 ][256] = {0};
 CCKD64_L2ENT  empty64_l2 [ CKD_NULLTRK_FMTMAX + 1 ][256] = {0};
 
+#if defined( __EMSCRIPTEN__ )
+/* Write-through for the browser build.                               */
+/* 1 = at the end of every channel program that updated a track, the  */
+/* updated track images of the device are handed to the writer and    */
+/* the channel program ends only after they are in the (shadow) file. */
+/* Without it an updated track stays in the CCKD cache until the next */
+/* hardener / garbage collector pass, so a browser tab that is closed */
+/* or reloaded loses the last seconds of writes, and a close during a */
+/* flush leaves a random subset of them, not in MVS's write order     */
+/* ($HASP096 DISASTROUS ERROR in a JES2 checkpoint after such a       */
+/* close). With it a closed tab is like a power cut between two       */
+/* channel programs. Set by wasm_main --cckd-wrthru (persistent       */
+/* shadows on OPFS). Default 0 = upstream behaviour.                  */
+/* cckd_wasm_wrthru_skip: device numbers that keep the cached         */
+/* behaviour (wasm_main --cckd-wrthru-skip=, e.g. the page volume:    */
+/* its slots are not used again after an IPL).                        */
+DLL_EXPORT int cckd_wasm_wrthru = 0;
+DLL_EXPORT U16 cckd_wasm_wrthru_skip[ 8 ];
+DLL_EXPORT int cckd_wasm_wrthru_nskip = 0;
+static int cckd_wasm_wrthru_dev( DEVBLK* dev )
+{
+    int i;
+    if (!cckd_wasm_wrthru) return 0;
+    for (i = 0; i < cckd_wasm_wrthru_nskip && i < 8; i++)
+        if (cckd_wasm_wrthru_skip[i] == dev->devnum) return 0;
+    return 1;
+}
+#endif
+
 DLL_EXPORT int gctab[5] =   /* Garbage Collection parameters */
 {
     4096,                   /* critical  50%   - 100%    */
@@ -552,6 +581,23 @@ CCKD_EXT       *cckd;                   /* -> cckd extension         */
         cache_unlock (CACHE_DEVBUF);
     }
 
+#if defined( __EMSCRIPTEN__ )
+    /* Write-through (see cckd_wasm_wrthru): write this device's updated */
+    /* tracks now and wait until the writer has written them.            */
+    if (cckd->updated && !cckd->merging && !cckd->stopping && cckd_wasm_wrthru_dev( dev ))
+    {
+        cckd_flush_cache (dev);
+        while (cckd->wrpending)
+        {
+            cckd->cckdwaiters++;
+            wait_condition (&cckd->cckdiocond, &cckd->cckdiolock);
+            cckd->cckdwaiters--;
+        }
+        if (cckd->cckdwaiters)
+            broadcast_condition (&cckd->cckdiocond);
+    }
+    else
+#endif
     /* Cause writers to start after first update */
     if (cckd->updated && (cckdblk.wrs == 0 || cckd->cckdwaiters != 0))
         cckd_flush_cache (dev);