Hello There,
Thanks for posting in a very detailed information. Your border collapsed because you have implemented the columns incorrectly. You must have a row container first before you can display a column. Please have your code updated and use this one instead:
<div align="center" class="textbox">
<div class="x-container">
<div class="x-column x-sm x-1-2" style="">
<p><a href="#colombiamust">Must Visit Destinations</a></p>
<p><a href="#colombiaunique">Unique Experiences</a></p>
<p><a href="#colombialesser">Lesser-Visited Destinations</a></p>
<p><a href="#colombiaonething">If you Only do One Thing</a></p>
<p><a href="#colombiafood">Colombian Food</a></p>
<p><a href="#colombiagbu">The Good, the Bad, the Ugly</a></p>
<p><a href="#colombiacosts">Costs of Travel</a></p>
</div>
<div class="x-column x-sm x-1-2 last" style="">
<p><a href="#colombiamoney">Money</a></p>
<p><a href="#colombiavisa">Visa Requirements</a></p>
<p><a href="#colombiagetaround">Getting Around</a></p>
<p><a href="#colombiaaccom">Accommodation</a></p>
<p><a href="#colombialanguage">Language</a></p>
<p><a href="#colombiaweather">Weather and Climate<</a></p>
<p><a href="#colombiahealth">Health and Safety</a></p>
</div>
</div>
</div>
We would loved to know if this has work for you. Thank you.