<BODY onLoad="javascript: document.forms[0].elements[0].focus();document.forms[0].elements[0].select();">
<FORM METHOD=POST ACTION="">
<INPUT TYPE="text" NAME="id" id="id" value="이메일주소를 입력하세요" >
</FORM>
</BODY>
팁! 이것만 알고가자/HTML_JAVASCRIPT
[JAVASCRIPT] 텍스트폼 전체 선택하기
<BODY onLoad="javascript: document.forms[0].elements[0].focus();document.forms[0].elements[0].select();">
<FORM METHOD=POST ACTION="">
<INPUT TYPE="text" NAME="id" id="id" value="이메일주소를 입력하세요" >
</FORM>
</BODY>