POST from an <A> tag

Not really, no. You can, however, do something like this:

<form action="theUrl" method="POST">

<input type="hidden" name="param1" value="val" />
<input type="hidden" name="param2" value="val2" />

<a href="#" onclick="this.parentNode.submit()">Go to that link!</a>

</form>

You should find a better way, though. This one does not degrade gracefully.

There is no way to POST an a element using only HTML.

As can be seen from this DTD fragment (HTML 4.01 spec):

<!ELEMENT A - - (%inline;)* -(A)       -- anchor -->
<!ATTLIST A
  %attrs;                              -- %coreattrs, %i18n, %events --
  charset     %Charset;      #IMPLIED  -- char encoding of linked resource --
  type        %ContentType;  #IMPLIED  -- advisory content type --
  name        CDATA          #IMPLIED  -- named link end --
  href        %URI;          #IMPLIED  -- URI for linked resource --
  hreflang    %LanguageCode; #IMPLIED  -- language code --
  rel         %LinkTypes;    #IMPLIED  -- forward link types --
  rev         %LinkTypes;    #IMPLIED  -- reverse link types --
  accesskey   %Character;    #IMPLIED  -- accessibility key character --
  shape       %Shape;        rect      -- for use with client-side image maps --
  coords      %Coords;       #IMPLIED  -- for use with client-side image maps --
  tabindex    NUMBER         #IMPLIED  -- position in tabbing order --
  onfocus     %Script;       #IMPLIED  -- the element got the focus --
  onblur      %Script;       #IMPLIED  -- the element lost the focus --
  >

There is no attribute that controls whether to use POST or GET with an a element.

You have to script it, if you want to abuse the semantics.

You can use CSS to make an <input type="submit"> look like a hyperlink.