The assets bundle system allows to generate `<script>` elements whose
resource is meant to be lazy loaded. In that case, a "data-src" is used
instead of "src" directly as attribute. This is valid HTML code: "src"
is not required. However... this lazy loading system also automatically
added `defer="defer"` which makes the "src" attribute required, thus
failing W3C validation.
Now, we only add "data-src" and the `defer="defer"` part is added on the
client-side, once the "data-src" is switched to "src". Note that this
"defer" attribute may not be needed in this case at all, but it cannot
hurt.
Part-of: odoo/odoo#120311
qsm-odoo
1b428bd0b0