With hdtv the borders are disappearing, but studios know the overscan area is traditionally removed by tv sets, so the keep anything important out of this area anyway.
Watch BBC breakfast any morning ( need to double check now they're at Salford Media Centre) – but you'll see blanking coming and going as they select different sources – I suspect some sources may still be being ARC'ed into (almost) widescreen. whilst other fill the screen.
The vertical drop, and showing either timecode or widescreen-switching signal is a different effect … frequently seen in the past with news editing on Betacam's and the playout machine still being locked to its input (as when editing) … and therefore the output delayed a couple of lines - but still seen each day on Breakfast/opt outs.
In the mixed world of 4x3 (12x9) and 16x9 there is also '14x9' viewed on either 16x9 or 12x9 screens, let alone all the times that '4x3' channels show widescreen 16x9 (or 14x9 copies?) in differing ratios including letterboxed.. and which are then viewed on 16x9 receivers; Within any / all of those are 'standards' for 'safe action' and 'safe caption' areas. Adverts also have a requirement of a minimum line-height for 'disclaimer' text etc, I believe …. (11 scan lines???)
And that's ignoring the 'Smart' (shades of 1984?) and other distorting modes used by TV manufacturers to fill the screens. Earlier someone mentioned the 'Auto' mode, as used in a Philips TV we have some years ago – it could, if allowed, auto-change after measuring the blanking at top and bottom of the screen … about 20-30 seconds after returning fom the adverts, it would 'correct' itself …. when playing back a 21x9 cinema-film, it would try to zoom it up to fill the screen, and off VHS (a long time ago) it looked a Very Horrible System.
Historical Note: Although 16x9 should be considered the 'new standard', and 4x3 its predecessor; tv pictures used to be 5x4 in some monochome days …. but since sets were all analogue in those days, the height control was a fairly common user control.
TEXT SIZE ON PI: The default, I believe, is a standard 8x8 matrix, with a possible Linux font option of 8x 14high; therefore on the default 1080line display this will be small!, and the video is 'unknown' – we don't even know if its a proper interlaced 625line display (unlikely), or 2 non-interlaced fields as typical on most home computers (except the Model B Mode 7 teletext, which was interlaced).