Apparently while validating an HTML5 document, the validator throws an error on the placeholder atribute for the element input. And after that shows the "Attributes for element input" and it is possible to see the placeholder atribute in that list.

Error: Attribute placeholder not allowed on element input at this point.
From line 37, column 1; to line 37, column 129
:</label>↩<input type="number" min="0" max="100" step="0.01" name="imposto" value="21.5" onchange="" class="variable" placeholder="21.5" /> % ↩<b

Attributes for element input:
Global attributes
accept
alt
autocomplete
autofocus
checked
dirname
disabled
form
formaction
formenctype
formmethod
formnovalidate
formtarget
height
list
max
maxlength
min
multiple
name
pattern
placeholder
readonly
required
size
src
step
type
value
width