Send HTML email from Google Apps Script. Full article here: http://blog.gsmart.in/google-apps-script-send-html-email

send-html-email-apps-script.js

function onOpen() 
{
  var ui = SpreadsheetApp.getUi();
  ui.createMenu('Process')
      .addItem('Approve', 'doApprove')
      .addToUi();
}

function doApprove()
{
   var cell = SpreadsheetApp.getActiveSheet().getActiveCell();
   var row = cell.getRow();
  
   var candidate = getCandidateFromRow(row); 
  
   var ui = SpreadsheetApp.getUi();
   var response = ui.alert('Approve '+candidate.name+'?', ui.ButtonSet.YES_NO);
  
   if(response == ui.Button.YES)
   {
     handleApproval(row, candidate);
   }
}

function getCandidateFromRow(row)
{
  var values = SpreadsheetApp.getActiveSheet().getRange(row, 1,row,3).getValues();
  var rec = values[0];
  
  var candidate = 
      {
        first_name: rec[0],
        last_name : rec[1],
        email: rec[2] 
      };
  
   candidate.name = candidate.first_name+' '+candidate.last_name;
  
   return candidate;
}

function handleApproval(row, candidate)
{
  var templ = HtmlService
      .createTemplateFromFile('candidate-email');
  
  templ.candidate = candidate;
  
  var message = templ.evaluate().getContent();
  
  MailApp.sendEmail({
    to: candidate.email,
    subject: "Your application is approved!",
    htmlBody: message
  });
  
  SpreadsheetApp.getActiveSheet().getRange(row, 4).setValue('approved');
  
}