The following HTML code contains non-breaking space in Figure 1:
<!DOCTYPE html>
<html lang='en-US' xml:lang='en-US'>
<head>
<meta charset='utf-8' />
</head><body>
<figcaption class='caption'>Figure 1:</figcaption><!-- tex4ht:label?: x1-2r -->
</body>
</html>
The compilation with sp html filename.html fails with the following message:
Error: [page 1] [string "csshtmltree = { typ = 'csshtmltree',..."]:11: invalid escape sequence near '"Figure