How to retain state of dropdown in html

hi all i have dropdown with options 10,20,30 and given on change ="this.form.submit" so what i want is if the form reloads i want to maintain what i have selected in dropdown suppose if i have selected 20 it has to 20 in dropdown even if form is submitted below is my drop down

<select id="dropsel" name="dropdel" onchange="this.form.submit()" >
<option value="select">-- Select --</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
<option value="5">5</option>
<option value="6">6</option>