diff --git a/web/source/kmwexthtml.ts b/web/source/kmwexthtml.ts
index 643178dd77..d83ad8912d 100644
--- a/web/source/kmwexthtml.ts
+++ b/web/source/kmwexthtml.ts
@@ -1,3 +1,8 @@
+interface Window {
+    attachEvent: (eventNameWithOn:string, callback: () => void) => void,
+    clientWidth: number
+}
+
 interface Document {
     selection: any                      // Is IE only; TS ignores it because of this.
 }
diff --git a/web/source/kmwuibutton.js b/web/source/kmwuibutton.ts
similarity index 99%
rename from web/source/kmwuibutton.js
rename to web/source/kmwuibutton.ts
index 0bf8a8bcd1..43bd5f8da3 100644
--- a/web/source/kmwuibutton.js
+++ b/web/source/kmwuibutton.ts
@@ -412,13 +412,13 @@ if(!window['keyman']['ui']['name']) {
       if(!ui.init) return;
   
       // Clear existing list first (first two nodes must be preserved)
-      for(var i=ui._KeymanWeb_KbdList.childNodes.length; i>2; i--)
+      for(var i:number=ui._KeymanWeb_KbdList.childNodes.length; i>2; i--)
         ui._KeymanWeb_KbdList.removeChild(ui._KeymanWeb_KbdList.childNodes[i-1]);
       
       var kbds=keymanweb['getKeyboards']();
       if(kbds.length > 0)
       {
-        for(var i=0; i<kbds.length; i++)
+        for(var i:number=0; i<kbds.length; i++)
           ui.registerKeyboard(kbds[i]['InternalName'],kbds[i]['LanguageName'],kbds[i]['Name'],kbds[i]['LanguageCode'],kbds[i]['RegionCode']);
       }
     } 
diff --git a/web/source/kmwuifloat.js b/web/source/kmwuifloat.ts
similarity index 99%
rename from web/source/kmwuifloat.js
rename to web/source/kmwuifloat.ts
index d6d3f5e203..e9ed686b54 100644
--- a/web/source/kmwuifloat.js
+++ b/web/source/kmwuifloat.ts
@@ -1,3 +1,5 @@
+/// <reference path="kmwexthtml.ts" />  // Includes KMW-added property declaration extensions for HTML elements.
+
 /***
    KeymanWeb 10.0
    Copyright 2017 SIL International
diff --git a/web/source/kmwuitoggle.js b/web/source/kmwuitoggle.ts
similarity index 98%
rename from web/source/kmwuitoggle.js
rename to web/source/kmwuitoggle.ts
index bab841e989..db999135cf 100644
--- a/web/source/kmwuitoggle.js
+++ b/web/source/kmwuitoggle.ts
@@ -1,3 +1,5 @@
+/// <reference path="kmwexthtml.ts" />  // Includes KMW-added property declaration extensions for HTML elements.
+
 /***
    KeymanWeb 10.0
    Copyright 2017 SIL International
@@ -32,7 +34,7 @@ if(!window['keyman']['ui']['name']) {
     if(util['isTouchDevice']()) throw '';
 
     // Initialize user interface common variables       
-    var ui=keymanweb['ui'] = {
+    var ui:any=keymanweb['ui'] = {
       name: 'toggle',
       initialized: false,
       controller: null,
@@ -460,7 +462,7 @@ if(!window['keyman']['ui']['name']) {
       if(_kbds.length > 1)
       {
         // Multiple keyboards
-        var _kmw_ctrl_img=document.getElementById('KMW_Controller_Img')
+        var _kmw_ctrl_img=<HTMLImageElement> document.getElementById('KMW_Controller_Img')
         _kmw_ctrl_img.src = imgPath+'kmw_logo_16_down.gif';
         _kmw_ctrl_img.style.width = '100%';
             
@@ -482,7 +484,7 @@ if(!window['keyman']['ui']['name']) {
       // Single keyboard
       else if(_kbds.length == 1)
       {
-        var _kmw_ctrl_img=document.getElementById('KMW_Controller_Img')
+        var _kmw_ctrl_img=<HTMLImageElement> document.getElementById('KMW_Controller_Img')
         _kmw_ctrl_img.src = imgPath+'kmw_logo_16.gif';
         
         ui.kbdButton.getElem().id = 'kmwico';
diff --git a/web/source/kmwuitoolbar.js b/web/source/kmwuitoolbar.ts
similarity index 99%
rename from web/source/kmwuitoolbar.js
rename to web/source/kmwuitoolbar.ts
index 5965491f7a..66f7ecbf1e 100644
--- a/web/source/kmwuitoolbar.js
+++ b/web/source/kmwuitoolbar.ts
@@ -326,7 +326,7 @@ if(!window['keyman']['ui']['name']) {
           languageCode = Keyboards[j]['LanguageCode'];
           max++;
         }   
-        max = ((max+3)/4).toFixed(0);   // Get number of entries per column
+        max = Number(((max+3)/4).toFixed(0));   // Get number of entries per column
   
         // Add language list to columns for the region
         languageCode='';
diff --git a/web/source/tsconfig.base.json b/web/source/tsconfig.base.json
new file mode 100644
index 0000000000..7b892a7f12
--- /dev/null
+++ b/web/source/tsconfig.base.json
@@ -0,0 +1,9 @@
+{
+  "compilerOptions": {
+	"allowJs": true,
+	"module": "system",
+	"outDir": "../output/",
+	"sourceMap": true,
+	"target": "es5"
+  }
+}
diff --git a/web/source/tsconfig.json b/web/source/tsconfig.json
index 32611c32d9..0138b46b99 100644
--- a/web/source/tsconfig.json
+++ b/web/source/tsconfig.json
@@ -1,19 +1,3 @@
 {
-  "compilerOptions": {
-	"allowJs": true,
-	"module": "system",
-	"outFile": "../output/keymanweb.js",
-	"sourceMap": true,
-	"target": "es5"
-  },
-  "files": [
-	"kmwbase.ts",
-	"keymanweb.ts",
-	"kmwosk.ts", 
-	"kmwnative.ts", 
-	"kmwcallback.ts",
-	"kmwkeymaps.ts",
-	"kmwlayout.ts",
-	"kmwinit.ts"
-  ]
+	"extends": "./tsconfig.web.json"
 }
diff --git a/web/source/tsconfig.ui.json b/web/source/tsconfig.ui.json
new file mode 100644
index 0000000000..8925b203a2
--- /dev/null
+++ b/web/source/tsconfig.ui.json
@@ -0,0 +1,9 @@
+{
+	"extends": "./tsconfig.base.json",
+  "files": [
+		"kmwuibutton.ts",
+		"kmwuifloat.ts",
+		"kmwuitoggle.ts", 
+		"kmwuitoolbar.ts"
+  ]
+}
diff --git a/web/source/tsconfig.web.json b/web/source/tsconfig.web.json
new file mode 100644
index 0000000000..f383773fce
--- /dev/null
+++ b/web/source/tsconfig.web.json
@@ -0,0 +1,16 @@
+{
+	"extends": "./tsconfig.base.json",
+  "compilerOptions": {
+		"outFile": "../output/keymanweb.js"
+  },
+  "files": [
+		"kmwbase.ts",
+		"keymanweb.ts",
+		"kmwosk.ts", 
+		"kmwnative.ts", 
+		"kmwcallback.ts",
+		"kmwkeymaps.ts",
+		"kmwlayout.ts",
+		"kmwinit.ts"
+  ]
+}