(html5)请问为何我的按钮无法点击
"单人战役"和"称霸网络"这两个文本按钮 谢谢帮助<body>
<div id = "gamecontainer">
<div id = "gamestartscreen" class = "gameplayer">
<span id = "singleplayer" onclick = "singleplayer.start();">单人战役</span><br>
<span id = "multiplayer" onclick = "multiplayer.start();">称霸网络</span><br>
</div>
<div id = "missionscreen" class = "gamelayer">
<input type = "button" id = "entermission" onclick = "singleplayer.play();">
<input type = "button" id = "exitmission" onclick = "singleplayer.exit();">
<div id = "missonbriefing">欢迎来到你的第一关
</div>
</div>
<div id = "gameinterfacescreen" class = "gameplayer">
<div id = "gamemessages"></div>
<div id = "callerpicture"></div>
<div id = "cash"></div>
<div id = "sidebarbuttons"></div>
<canvas id = "gamebackgroundcanvas" height = "400" width = "480"></canvas>
<canvas id = "gameforegroudcanvas" height = "400" width = "480"></canvas>
</div>
<div id = "loadingscreen" class = "gamelayer">
<div id = "loadingmessage"></div>
</div>
</div>
</body>
