The actress Marlee Matlin shimmied her way onto “Dancing With the Stars” two years ago, memorably using sign language to tell viewers to “read my hips.” But when Ms. Matlin, who is deaf, went to ABC.com to watch a replay of the show, she was impeded because the network’s videos were missing captions.

Closed-captioning is mandatory on television, but not for TV programs on the Internet. And that has turned Web sites like ABC.com into battlegrounds for advocates like Ms. Matlin, who have spoken up on the lack of captions on sites like CNN.com and services like Netflix.

Media companies say they are working hard to make online video more accessible. YouTube, the world’s biggest video Web site by far, now supplies mostly accurate captions using voice-recognition software. ESPN is offering captions for its live streams of World Cup matches. And ABC now applies the TV captions for “Dancing With the Stars” to ABC.com.

But big gaps remain much to the dismay of deaf Web users. Television episodes on CBS.com, news videos on CNN.com and entertainment clips on MSN.com all lack captions, to name a few.