just a note about native multiple select.

[jimmywarting]

components/src/material/select/select.md

Lines 94 to 96 in 35d37f6

	Using multiple selection with a native select element (`<select multiple>`) is discouraged
	inside `<mat-form-field>`, as the inline listbox appearance is inconsistent with other
	Material Design components.

they can look consistent IF you add size attribute.

size
If the control is presented as a scrolling list box (e.g., when multiple is specified), this attribute represents the number of rows in the list that should be visible at one time. Browsers are not required to present a select element as a scrolled list box. The default value is 0.

Note: According to the HTML specification, the default value for size should be 1; however, in practice, this has been found to break some websites, and no other browser currently does that, so Mozilla has opted to continue to return 0 for the time being with Firefox.

if someone wants to use native multiple select, then they can do that but it should be recommended that size attribute also gets added if doing so.