**Similar example <label for=...>
Property and attribute aren’t always 1:1. An often encountered example is the label tag
<label for="someId">
In Angular
<label [for]="someId">
fails with the same error and you’d need to bind like
<label attr.for="{{someId}}">
or
<label [attr.for]="someId">
but
<label [htmlFor]="someId">
would also work because in this case htmlFor is the property that is reflected to the DOM for attribute.
See also https://developer.mozilla.org/de/docs/Web/API/HTMLLabelElement for the htmlFor property (in the Properties section)
See also What is the difference between attribute and property?
colSpan the actual property name
According to https://developer.mozilla.org/en-US/docs/Web/API/HTMLTableCellElement colSpan is the property that is reflected to the colspan attribute therefore (uppercase S)
<td [colSpan]="1 + 1">Column</td>
See also https://plnkr.co/edit/BZelYOraELdprw5GMKPr?p=preview
works just fine.
Why does Angular bind to properties by default
Angular binds to the property by default for performance reasons. Binding to an attribute is expensive because attributes are reflected in the DOM and a change in the DOM can causes reevaluation of CSS styles that might match after the change, while properties are just a value in a JavaScript object that changed.
With attr. you opt in explicitely to the expensive behavior.