Org mode tables are exported to HTML using the table attributes defined in
org-html-table-default-attributes. The default setting makes tables
without cell borders and frame. If you would like to change this for
individual tables, place something like the following before the table:
#+CAPTION: This is a table with lines around and between cells #+ATTR_HTML: :border 2 :rules all :frame border
You can also modify the default tags used for each row by setting
org-html-table-row-tags. See the docstring for an example on
how to use this option.