Please read the following table in HTML format from a webpage and then answer the question in detail according to the table. Each table cell is defined by a <td> and a </td> tag. Each table row starts with a <tr> and ends with a </tr> tag. 'rowspan=m' and 'colspan=n' represents a cell spaning over 'm' rows and 'n' columns.

Webpage title:
1968 Greek constitutional referendum
Table title:
Results
Table:
<table id='hittask' contenteditable='False'>
<tr> <th colspan=1 rowspan=1 > Choice </th><th colspan=1 rowspan=1 > Votes </th><th colspan=1 rowspan=1 > % </th></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > For </td><td colspan=1 rowspan=1 > 4,713,421 </td><td class="highlighted"  colspan=1 rowspan=1 > 92.1 </td></tr>
<tr> <td colspan=1 rowspan=1 > Against </td><td colspan=1 rowspan=1 > 403,829 </td><td colspan=1 rowspan=1 > 7.9 </td></tr>
<tr> <td colspan=1 rowspan=1 > Invalid/blank votes </td><td colspan=1 rowspan=1 > 16,656 </td><td colspan=1 rowspan=1 > – </td></tr>
<tr> <td colspan=1 rowspan=1 > Total </td><td colspan=1 rowspan=1 > 5,133,906 </td><td colspan=1 rowspan=1 > 100 </td></tr>
<tr> <td class="highlighted"  colspan=1 rowspan=1 > Registered voters/turnout </td><td colspan=1 rowspan=1 > 6,606,111 </td><td class="highlighted"  colspan=1 rowspan=1 > 77.7 </td></tr>
<tr> <td colspan=3 rowspan=1 > Source: Nohlen & Stöver </td></tr>
</table>
Question:
What was the outcome of the 1968 Greek constitutional referendum?
Answer:
The 1968 Greek constitutional referendum was approved by 92.1% of voters with a voter turnout of 77.7%.