Important notice, whenever you use HTML5 client be aware that you use browser with functionally limited sandbox environment which can't be overriden, unconsidered situations of browser vulnerabilities but not of our interests.
There are certain situations where you may need to use shortcuts but due nature of browsers (read notice above) several shortcuts are intercepted on system level and fire either no action at all, or are not forwarded to DOM level of browser event model due predefined browser actions. Since this is not application bug but functional limitation of browsers you have the choice either to accept this limitation, or switch to native RDP client, or use shortcuts menu as last resort.
1. To open this menu press following key sequence: Ctrl down > Left_Windows down > Left_Windows up > Ctrl up
Important: Ctrl has to be released after Left_Windows key was released, on Mac instead Left_Windows key use Left_Cmd
2.1 Backspace or Del button or click with left mouse on any area around will close menu without future action.
2.2 Up/Down keys will set focus on wished element but not yet execute it.
2.3 Enter/Space key will execute focused element, additionally any click with mouse on element will focus and execute it. After execution if the sequence doesn't contain *-Half key this will auto close the menu.
2.4 Combinations of Shift + Alt + * will focus and execute wished element directly without using mouse. In each menu line by right side you will see representative key * for combination sequence Shift + Alt + *
As example * = "W", also Shift + Alt + W which stands for Ctrl + W etc.
3. Combinations with *-Half inside combo, also Ctrl-Half or Alt-Half will not auto close the menu after execution but just execute the combination without releasing mentioned *-Half key, if Half key is Ctrl then Ctrl will be hold while sequence key Tab pressed, and same counts for Alt-Half key.
So, if you need Alt + Tab sequence to be pressed only by keyboard without mouse, firstly you need to call the menu by
3. A: Ctrl down > Left_Windows down > Left_Windows up > Ctrl up
3. B: Shift down > Alt down > M (press several times for switching) < M up < Alt up < Shift up
or alternatively press Up key and then Enter button or Space button to repeat
3. C: finally press Backspace or Delete button which will release holded *-Half key, depending on action Ctrl or Alt, and only afterwards the menu will be finally closed.