brainfuck, 145 bytes
+++++++++[>++++++++++<-]>+[>+>+<<-]>>++>,>+++++++++[<----->-]<--->>>,>+++++++++[<----->-]<--->++++++++++<[<<<[>+>+<<-]>[<<<.>.>>-]>[<<+>>-]>>.<-]
Experimente online!
Meu primeiro código de golfe! Yay!
A entrada está em ascii + 48, portanto, para fazer 50, 50, você deve inserir b, b (as letras ascii para 98)
Explicação
+++++++++[>++++++++++<-]>+ Get the opening square bracket into first position
[>+>+<<-] Get it into the second and third position
>>++ Get the third position to be the closing bracket
>
,>+++++++++[<----->-]<--- Get first number into fourth cell
>>>
,>+++++++++[<----->-]<--- Get second number into seventh cell
>++++++++++ get newline into 8th position
<
[ Start our height loop
<<<[>+>+<<-] Get the width into the fifth and sixth positions
>[ Start our width loop at the fifth position
<<<.>. Print the second and third positions
>>-] Decrement the fifth position
>
[<<+>>-] copy the sixth position into the fourth position
>>. print newline
<-]