Some more pizzaz.

www/main.js

 const API_PREFIX = "https://blockstream.info/api";
 
 const IRON_BLOCK = "https://gamepedia.cursecdn.com/minecraft_gamepedia/7/7e/Block_of_Iron_JE4_BE3.png?version=692673bafa1e94785ab6012d7a3c8dc4";
-const GOLD_BLOCK = "https://gamepedia.cursecdn.com/minecraft_gamepedia/7/72/Block_of_Gold_JE6_BE3.png?version=9d1a63c717df80feffa9ec93ebceb014";
-const DIAMOND_BLOCK = "https://gamepedia.cursecdn.com/minecraft_gamepedia/6/6b/Block_of_Diamond_JE6_BE3.png?version=117daf438c2ddfff59bab05d5715f6d2";
+const GOLD_BLOCK = "https://gamepedia.cursecdn.com/minecraft_gamepedia/archive/7/72/20190429060628%21Block_of_Gold_JE6_BE3.png?version=448d791eb688910a0cc215181312b715";
+const DIAMOND_BLOCK = "https://gamepedia.cursecdn.com/minecraft_gamepedia/archive/6/6b/20190502005227%21Block_of_Diamond_JE6_BE3.png?version=721888d830050c06303696cf79695eb9";
 
-var nextHalvingHeight = 630000;
+const HALVING_PERIOD = 210000;
 
-var curTipBlock = null;
 var recentBlocks = null;
 
+function getCurTipBlock() {
+    return recentBlocks[0];
+}
+
 function doApiReq(endpoint, cb) {
     let xhr = new XMLHttpRequest();
 
 }
 
 function doGetBlockCoinBaseTx(blockhash, cb) {
-    doApiReq("/block/" + blockhash + "/txs", function(resp) {
+    let reqSuffix = "/block/" + blockhash + "/txs";
+    let internalCb = function(resp) {
         if (resp != null) {
             cb(resp[0]);
         } else {
             cb(null);
         }
+    };
+
+    doApiReq(reqSuffix, function(resp) {
+        if (resp != null) {
+            internalCb(resp);
+        } else {
+            // Sometimes the data hasn't been fully processed yet by the time we
+            // make the call so wait a bit and then try again.
+            setTimeout(function() {
+                doApiReq(reqSuffix, internalCb);
+            }, 1000)
+        }
     });
 }
 
     }
 }
 
-const HALVING_PERIOD = 210000;
+function calcNextHalvingHeight() {
+    let height = recentBlocks[0].height;
+    let epoch = Math.floor(height / HALVING_PERIOD);
+    return (epoch + 1) * HALVING_PERIOD;
+}
 
 function calcRewardAtHeight(height) {
     let epoch = Math.floor(height / HALVING_PERIOD);
     return datePart + " " + timePart;
 }
 
+function isHalvingHeight(height) {
+    return height % HALVING_PERIOD == 0;
+}
+
 function makeBlockElem(block, prevBlock) {
     let entry = document.createElement("div");
 
     let iconElem = document.createElement("img");
     iconElem.classList.add("blbicon");
     iconCtr.appendChild(iconElem);
-    if (block.height % HALVING_PERIOD == 0) {
+    if (isHalvingHeight(block.height)) {
         iconElem.src = DIAMOND_BLOCK;
+        entry.classList.add("halvingblockentry");
     } else {
         iconElem.src = GOLD_BLOCK;
     }
 var timeLeftElem = null;
 var halfTimeElem = null;
 
+const TARGET_BLOCKTIME = 10 * 60;
+var blocktimeAvgSecs = TARGET_BLOCKTIME;
+
 function updateRemainingCount(block) {
     if (blocksLeftElem == null) {
         blocksLeftElem = document.getElementById("blocksleft");
         halfTimeElem = document.getElementById("halftime");
     }
 
-    let blocksLeft = nextHalvingHeight - curTipBlock.height;
+    let nextHalvingHeight = calcNextHalvingHeight();
+    let blocksLeft = nextHalvingHeight - getCurTipBlock().height;
     blocksLeftElem.innerHTML = blocksLeft.toString()
 
-    if (blocksLeft < 1) {
+    if (blocksLeft <= 1) {
         timeLeftElem.innerHTML = "Halving imminent!";
 
         // Hide the expected time.
         let nowUnix = Date.now() / 1000; // wtf???
 
         let sinceLastBlock = nowUnix - block.timestamp;
-        let secsLeft = ((blocksLeft * 10 * 60) - sinceLastBlock)|0;
+        let secsLeft = ((blocksLeft * blocktimeAvgSecs) - sinceLastBlock)|0;
 
         let secsPart = secsLeft % 60;
         let minLeft = (secsLeft - secsPart) / 60;
 
 function addNewBlock(block) {
 
-    let prev = recentBlocks[0];
+    let prev = getCurTipBlock();
     recentBlocks = [block].concat(recentBlocks);
+    //updateBlocktimeAvg();
     let elem = makeBlockElem(block, prev);
 
     elem.classList.add("newblock");
     let blocklist = document.getElementById("blocklist");
     let loadingElem = document.getElementById("blocklistloading");
 
-    curTipBlock = blocks[0];
-    updateRemainingCount(curTipBlock.height);
+    updateRemainingCount(getCurTipBlock());
 
     for (let i = 0; i < blocks.length; i++) {
         let block = blocks[i];
     }
 
     loadingElem.style.display = "none";
-    updateRemainingCount(curTipBlock);
+    //updateRemainingCount(curTipBlock);
 }
 
 function getPollDelayAtHeight(height) {
     let blocksUntilHalving = HALVING_PERIOD - (height % HALVING_PERIOD);
-    if (blocksUntilHalving == 1) {
+    if (blocksUntilHalving <= 1) {
         return 1000;
     } else if (blocksUntilHalving == 2) {
         return 2500;
             return;
         }
 
+        let curTip = getCurTipBlock();
+
         // Find the index of the newest known block.
-        let knownHeight = recentBlocks[0].height;
+        let knownHeight = curTip.height;
         let newestKnownIndex = -1;
         for (let i = 0; i < blocks.length; i++) {
             if (blocks[i].height == knownHeight) {
         }
 
         recentBlocks = blocks;
+        //updateBlocktimeAvg();
 
         populateRecentBlocks(blocks);
         setTimeout(checkNewBlocks, 5000);

www/style.css

     }
 }
 
-.closebtn {
-    width: 36pt;
-    margin: 4pt;
-
-    border: 1px solid grey;
-    border-radius: 4px 4px 4px 4px;
-    padding: 4pt;
+.halvingblockentry .entryinner {
+    animation: 0.5s linear 0s infinite alternate both running halvingblockanim;
+}
 
-    background-color: lightgrey;
-    text-align: center;
+@keyframes halvingblockanim {
+    from {
+        background-color: #90ffff;
+    }
+    to {
+        background-color: #eeffff;
+    }
 }