A, as follows
Second, before clicking
Third, click
<! DOCTYPE HTML>
<HTML>
<Meta HTTP-equiv = "Content-Type" Content = "text / HTML; charset = UTF-8" />
<body>
<h2> JavaScript can do </ h2>
<the p- id = "demo"> JavaScript can change the style of HTML elements. </ the p->
<the Button of the type = "the Button" onclick = "document.getElementById ( 'Demo'). style.fontSize = '35px'">
Click me!
</ Button>
</ body>
</ HTML>